Abu Bakr: "Kill them by every means, by fire or whatever else... if they deny payment of the alms tax, raid them without any warning."
Tarikh al-Tabari, Vol. 10 Page 100
Narrated Suwayd b. al-Math'abah al-Riyahi:

Khalid b. al-Walid arrived in al-Butah, whereupon he found no one there; he found that Malik had dispersed them among their flocks and forbidden them to gather when he became perplexed over his situation. [In doing so] he said, "Oh Banu Yarbu`, we have disobeyed our commanders, inasmuch as they called us to this religion, whereas the people held us back from it, so that we have neither prospered nor succeeded. I have reconsidered this situation and found it feasible for them without any managing (siyasah). For lo, the situation is one that the people do not manage. Beware of acting in a hostile manner against a group having a mandate [from God to exercise authority]1; so disperse to your territories and enter into this cause." So they dispersed accordingly to their flocks, and Malik went out until he returned to his residence.

 

When Khalid reached al-Butah, he scattered portions of the army and ordered them to summon [the people] to Islam, and to bring to it whoever had not [yet] responded; and if he resisted, to kill him. [This] was part of that with which Abu Bakr had charged him "When you encamp someplace, make the call to prayer and the iqamah. Then, if the people make the call to prayer and the iqamah, leave them alone; but if they do not do so, there is no [course] but to raid them. [In that case] kill them by every means, by fire or whatever else. And if they respond to you in the call to Islam, then question them [further]; if they affirm [payment of] the alms tax, then accept that from them; but if they deny it, then there is no [course] but to raid them without any word [of warning]."

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 10, Page 100
Arabic reference  : 
Narrated by: Abu Ja`far, from al-Sari b. Yahya, from Shu`ayb b. Ibrahim, from Sayf b. `Umar, from Khuzaymah b. Shajarah al-`Uqfani, from `Uthman b. Suwayd, from Suwayd b. al-Math'abah al-Riyahi.

Note: 1. Umar ibn Khattab, when explaining Saqifah, describes the assention to khilfah as "sudden" but Allah (swt) "saved them from its bad consequnces", and hence, nobody would reasonably describe the Khilafah of Abu Bakr a mandate from Allah (swt) to excecise authority. We can hence conclude he is speaking of Ali, who did make such a claim, as is evidenced by the event of the Nomination of Uthman

 
Quran 16:125: Abu Bakr's instructions and Khalid's actions directly oppose the instructions of the Quran on how to deal with non believers.

Reference: Quran: An-Nahl 16:125

Quran: An-Nahl 16:125
Translation: Dr. Mustafa Khattab, the Clear Quran

Invite ˹all˺ to the Way of your Lord with wisdom and kind advice, and only debate with them in the best manner. Surely your Lord ˹alone˺ knows best who has strayed from His Way and who is ˹rightly˺ guided.

ٱدْعُ إِلَىٰ سَبِيلِ رَبِّكَ بِٱلْحِكْمَةِ وَٱلْمَوْعِظَةِ ٱلْحَسَنَةِ ۖ وَجَدِلْهُم بِٱلَّتِى هِىَ أَحْسَنُ ۚ إِنَّ رَبَّكَ هُوَ أَعْلَمُ بِمَن ضَلَّ عَن سَبِيلِهِۦ ۖ وَهُوَ أَعْلَمُ بِٱلْمُهْتَدِينَ

Reference  : Quran: An-Nahl 16:125
 
Quran 2:256: Murdering non believers is Haram.
Quran: Al-Baqarah 2:256
Translation: Yusuf Ali

Let there be no compulsion in religion: Truth stands out clear from Error: whoever rejects evil and believes in Allah hath grasped the most trustworthy hand-hold, that never breaks. And Allah heareth and knoweth all things.

‏لَآ إِكْرَاهَ فِى ٱلدِّينِ ۖ قَد تَّبَيَّنَ ٱلرُّشْدُ مِنَ ٱلْغَىِّ ۚ فَمَن يَكْفُرْ بِٱلطَّغُوتِ وَيُؤْمِنۢ بِٱللَّهِ فَقَدِ ٱسْتَمْسَكَ بِٱلْعُرْوَةِ ٱلْوُثْقَىٰ لَا ٱنفِصَامَ لَهَا ۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌ ‎

Reference  : Quran: Al-Baqarah 2:256
 
The Prophet said: "Punishment with fire is done by none except Allah."

Reference: Sahih al-Bukhari 2954

Sahih al-Bukhari 2954
Narrated Az-Zuhri:

Allah's Messenger (ﷺ) sent us on military expedition telling us, "If you find such and such persons (he named two men from Quraish), burn them fire." Then we came to bid him farewell, when we wanted to set out, he said: "Previously I ordered you to burn so-and-so and so-and-so with fire, but as punishment with fire is done by none except Allah, if you capture them, kill them, (instead)."

وَقَالَ ابْنُ وَهْبٍ أَخْبَرَنِي عَمْرٌو، عَنْ بُكَيْرٍ، عَنْ سُلَيْمَانَ بْنِ يَسَارٍ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ أَنَّهُ قَالَ بَعَثَنَا رَسُولُ اللَّهِ صلى الله عليه وسلم فِي بَعْثٍ، وَقَالَ لَنَا ‏"‏ إِنْ لَقِيتُمْ فُلاَنًا وَفُلاَنًا ‏"‏‏.‏ ـ لِرَجُلَيْنِ مِنْ قُرَيْشٍ سَمَّاهُمَا ـ فَحَرِّقُوهُمَا بِالنَّارِ‏.‏ قَالَ ثُمَّ أَتَيْنَاهُ نُوَدِّعُهُ حِينَ أَرَدْنَا الْخُرُوجَ فَقَالَ ‏"‏ إِنِّي كُنْتُ أَمَرْتُكُمْ أَنْ تُحَرِّقُوا فُلاَنًا وَفُلاَنًا بِالنَّارِ، وَإِنَّ النَّارَ لاَ يُعَذِّبُ بِهَا إِلاَّ اللَّهُ، فَإِنْ أَخَذْتُمُوهُمَا فَاقْتُلُوهُمَا ‏"‏‏.‏

Reference  : Sahih al-Bukhari 2954
In-book reference  : Book 56, Hadith 166
 
Malik b. Nuwayrah and his tribe were Muslim, but were in opposition to Abu Bakr. Khalid's army hunted those in opposition. Khalid personally murdered Malik, and not only married Malik's wife that night, but also "leaped" upon her. Abu Bakr forgave him.
Tarikh al-Tabari, Vol. 10 Page 103
Narrated Talhah b. Abdallah b. Abd al-Rahman b. Abu Bakr al-Siddiq:

Among Abu Bakr's instructions to his armies was [this]: "When you come upon one of the peoples' abodes, and then hear the call to prayer in it, desist from its people until you have asked them for what reason they were hostile.1 But if you do not hear the call to prayer, then launch a raid such that you kill and burn." Among those who testified that Malik [b. Nuwayrah][had embraced] Islam was Abu Qatadah al-Harith b. Rib`i, a brother of Banu Salimah. He made a vow to God that he would never witness a war with Khalid b. al-Walid after that. He used to relate that when they came upon a group they would watch them under cover of night so that the group took up arms. "Then we would say, 'We are Muslims,"whereupon they would say, 'We too are Muslims.' [So] we would say, 'Then what is the meaning of your weapons?' They would say, 'And what is the meaning of your weapons?' whereupon we would say, 'If you are as you say, then put your weapons down.' So they would put them down; then we would pray and they would pray."

 

Khalid used to excuse himself for killing [Malik]2 [on the grounds] that [Malik] had said, when he was interrogating him, "I think your companion was only saying such and such." (Khalid) said, "And why didn't you reckon him a companion of yours?" Then he made him come forward and struck off his head and those of his companions. Then, when `Umar b. al-Khattab learned of their murder, he spoke of it with Abu Bakr repeatedly, saying, "The enemy of God transgressed against a Muslim man, killing him and then leaping upon his wife."3 Khalid b. al-Walid approached [Medina] on his return until he entered the mosque, wearing a robe of his on which was iron rust, and with his head wrapped in a turban of his in which arrows had become planted. So when he entered the mosque, `Umar went to him and pulled the arrows from his head and smashed them. Then he said, "What hypocrisy, to kill a Muslim man and then leap upon his wife! By God, I would pelt you with stones."4 Khalid b. al-Walid did not speak to him, and thought that Abu Bakr would only have the same opinion about him as Umar, until he entered upon Abu Bakr. When he entered upon him, he told him the story and Abu Bakr pardoned him and forgave him without punishment for whatever had happened in his recent campaign. So Khalid went out when Abu Bakr favored him. Umar was seated in the mosque, so he said, "Come to me, you son of the world! " From this, Umar knew that Abu Bakr had favored him, so he did not speak to him and went into his house.

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 10 Page 103
Arabic reference  : 
Narrated by: Ibn Humayd, from Salamah, from Mubammad b. Ishaq, from Talhah b. Abdallah b. Abd al-Rahman b. Abu Bakr al-Siddiq.

Note: 1. Clearly, there were Muslims that were hostile.

2. Khalid is implicated in murdering Malik bin Nuwayrah personally.

3. Umar admonishes Khalid for not only marrying the wife of Malik, but also leaping upon her.

4. The punishment for adultary.

5. If anyone else is to blame for Malik's murder, why is Umar angry with Khalid? And why does Abu Bakr forgive Khalid?, Yet there is no mention of anyone else being summoned, questioned, punished or forgiven.

 
The decapitated heads of the tribe, including Malik, were used to hold up pots of foods in a pit of fire.
Tarikh al-Tabari, Vol. 10 Page 103
Narrated Suwayd:

Malik b. Nuwayrah was one of the hairiest of people. Now the men of the army used the heads [of the slain captives] to hold up their cooking-pots, and there was no head among them whose skin the fire did not reach except Malik's; the pot became well-cooked but his head did not cook because of the amount of hair on it, the hair preventing [the fire's] heat from reaching the skin. Mutammim described him in verse, mentioning his slenderness. `Umar had seen him when he came to the Prophet, so he said, "Was he really like that, Oh Mutammim?" He replied, "As for what I said, yes."

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 10 Page 103
Arabic reference  : 
Narrated by: al-Sari, from Shu`ayb, from Sayf, from Khuzaymah, from `Uthman, from Suwayd.
 
Abu Qatadah testifies that Malik b. Nuwayrah and his tribe were Muslims, that they had made the Adhaan, the Iqamah and had performed salah that morning.
Tarikh al-Tabari, Vol 10. Page 101
Narrated Suwayd b. al-Math'abah al-Riyahi:

Then the cavalry brought Malik b. Nuwayrah to [Khalid], along with some people of Banu Tha'labah b. Yarbu', of 'Asim and `Ubayd and 'Arin and Ja`far. The raiding party disagreed about them; among them was Abu Qatadah, who was one of those who testified that they had made the call to prayer and the iqamah and had performed the prayer.

 

The people disagreed about them. Abu Qatadah said [to Khalid], "This is your doing." At this Khalid countered him with rough speech, whereupon (Abu Qatadah) became angry and proceeded to Abu Bakr. At this Abu Bakr became angry at (Abu Qatadah)1 until 'Umar spoke to him on his behalf, but (Abu Bakr) would only be content if (Abu Qatadah) returned to (Khalid); so [Abu Qatadah] returned to [Khalid], so that he came to Medina with him.

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol 10. Page 101
Arabic reference  : 
Narrated by: Abu Ja`far, from al-Sari b. Yahya, from Shu`ayb b. Ibrahim, from Sayf b. `Umar, from Khuzaymah b. Shajarah al-`Uqfani, from `Uthman b. Suwayd, from Suwayd b. al-Math'abah al-Riyahi.

Note: 1. Abu Bakr approves of the acts committed by Khalid, as he gets angry with Abu Qatadah for being in opposition to Khalid.

 
Corroborating testimony that Malik and his tribe were Muslims according to the test laid out by Abu Bakr.
Tarikh al-Tabari, Vol 10. Page 102
Narrated the father of Hisham b. 'Urwah:

A group from the raiding party testified that they [themselves] had made the call to prayer and done the iqamah and prayed, whereupon [the people being raided] had done likewise; whereas others testified that nothing like that had occurred, so that [those raided] were killed. [Malik's] brother' Mutammim b. Nuwayrah, came begging Abu Bakr for [permission to seek vengeance for] his blood and requesting him to make [the guilty parties] captive; but he wrote to him rejecting [their] capture. `Umar pressed him to dismiss Khalid, saying, "In his sword there really is forbidden behavior," whereupon hereplied, "Oh `Umar, I will not sheathe a sword that God has drawn against the unbelievers."

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol 10. Page 102
Arabic reference  : 
Narrated by: al-Sari, from Shu'ayb, from Sayf, from Hisham b. 'Urwah, from his father.
 
Corroborating account that Malik was attacked for being in opposition to Abu Bakr. Ansar asserted they did not have orders to attack Malik, but joined the raid for fear of **retribution**.
Tarikh al-Tabari, Vol. 10, page 99
Narrated Sahl-al-Qasim and `Amr b. Shu`ayb:

When Khalid wanted to march, he left Zafar mopping up Asad, Ghatafan, Tayyi', and Hawazin; so he marched heading for al-Butah this side of al-Hazn, while Malik b. Nuwayrah was there. His situation became doubtful to him, and the Ansar hesitated [to join] Khalid and held back from him saying, "These were not the caliph's orders to us; the caliph charged us to stand fast until he wrote to us after we had finished with al-Buzakhah and mopped up [opposition in] the people's territory." Whereupon Khalid said, "He did not charge you with this; rather, he charged me to proceed, and I am the commander and the one to whom communications come. Even if there reached me no letter nor any command of his, I would seize any opportunity that I perceived before informing him of it, if I thought that by informing him the opportunity would slip away from me. Likewise, if we were tempted by some situation about which we had no instructions from him, we would not fail to consider what was the most desirable option before us and then to act on it. Now this Malik b. Nuwayrah is right opposite us and I am heading for him with those who are with me of the Muhajirun and those who follow in good works; I shall not force you." Khalid proceeded and the Ansar repented and urged one another on, saying, "If the group achieves good [results], it is a good you will be excluded from; and if some misfortune befalls them, the people will shun you for it." So they agreed to join with Khalid, and dispatched messengers to him, whereupon he waited for them until they joined him. Then he marched until he reached al-Butah, but found no one there.

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 10, Page 99
Arabic reference  : 
Narrated by: al-Sari, from Shu`ayb, from Sayf, from Sahl-al-Qasim and `Amr b. Shu`ayb.
 
Corroborating account that Khalid b. Walid marries the wife of Malik that very night.
Tarikh al-Tabari, Vol. 10, page 102
Narrated Suwayd b. al-Math'abah al-Riyahi:

Khalid married Umm Tamim bt. al-Minhal (the wife of the slain Malik b. Nuwayrah), and abstaine from her so that the period between her menstruations should elapse. Now the Arabs used to find [the taking of] women abhorrent in war, and condemn it. 'Umar said to Abu Bakr, "In the sword of Khalid there really is forbidden behavior; and even if this [story about Malik's execution] were not true, it is necessary for you to take retaliation on him." He pestered him about that, but Abu Bakr did not take retaliation on [any of] his tax agents or commanders.1 Then he said, "Tell me, `Umar, (Khalid) sought to clear something up but [in the process] made a mistake; so stop berating him. (Abu Bakr) paid the blood price for Malik and wrote to Khalid to come before him; so he did that to explain his story, whereupon (Abu Bakr) pardoned him and accepted [his explanation]. But (Abu Bakr) did censure him over [his] marriage2 to one whom the Arabs considered it disgraceful [to marry] in that way.

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 10, page 102
Arabic reference  : 
Narrated by: Abu Ja`far, from al-Sari b. Yahya, from Shu`ayb b. Ibrahim, from Sayf b. `Umar, from Khuzaymah b. Shajarah al-`Uqfani, from `Uthman b. Suwayd, from Suwayd b. al-Math'abah al-Riyahi.

Note: 1. I.e. they operated with immunity.

2. Abu Bakr considered marrying the wife of Malik to be far worse than murdering him and cooking food with his decapitated head, as Abu Bakr censured Khalid for the former, and forgave the later.