Mu'awiyah was fortold to be inviting to hell fire.
Rasulullah (ﷺ) said: Ammar will be killed by a rebellious group, inviting to hell fire.

Reference: Sahih al-Bukhari 2812

Sahih al-Bukhari 2812
Narrated `Ikrima:

that Ibn `Abbas told him and `Ali bin `Abdullah to go to Abu Sa`id and listen to some of his narrations; So they both went (and saw) Abu Sa`id and his brother irrigating a garden belonging to them. When he saw them, he came up to them and sat down with his legs drawn up and wrapped in his garment and said, "(During the construction of the mosque of the Prophet) we carried the adobe of the mosque, one brick at a time while `Ammar used to carry two at a time. The Prophet (ﷺ) passed by `Ammar and removed the dust off his head and said, "May Allah be merciful to `Ammar. He will be killed by a rebellious aggressive group. `Ammar will invite them to (obey) Allah and they will invite him to the (Hell) fire."

حَدَّثَنَا إِبْرَاهِيمُ بْنُ مُوسَى، أَخْبَرَنَا عَبْدُ الْوَهَّابِ، حَدَّثَنَا خَالِدٌ، عَنْ عِكْرِمَةَ، أَنَّ ابْنَ عَبَّاسٍ، قَالَ لَهُ وَلِعَلِيِّ بْنِ عَبْدِ اللَّهِ ائْتِيَا أَبَا سَعِيدٍ فَاسْمَعَا مِنْ حَدِيثِهِ‏.‏ فَأَتَيْنَاهُ وَهُوَ وَأَخُوهُ فِي حَائِطٍ لَهُمَا يَسْقِيَانِهِ، فَلَمَّا رَآنَا جَاءَ فَاحْتَبَى وَجَلَسَ فَقَالَ كُنَّا نَنْقُلُ لَبِنَ الْمَسْجِدِ لَبِنَةً لَبِنَةً، وَكَانَ عَمَّارٌ يَنْقُلُ لَبِنَتَيْنِ لَبِنَتَيْنِ، فَمَرَّ بِهِ النَّبِيُّ صلى الله عليه وسلم وَمَسَحَ عَنْ رَأْسِهِ الْغُبَارَ وَقَالَ ‏ "‏ وَيْحَ عَمَّارٍ، تَقْتُلُهُ الْفِئَةُ الْبَاغِيَةُ، عَمَّارٌ يَدْعُوهُمْ إِلَى اللَّهِ وَيَدْعُونَهُ إِلَى النَّارِ ‏"‏‏.‏

Reference  : Sahih al-Bukhari 2812
In-book reference  : Book 56, Hadith 28
 
Rasulullah (ﷺ) said: "Rejoice, 'Ammar, the transgressing party shall kill you."
Jami` at-Tirmidhi 3800
Narrated Abu Hurairah:

that the Messenger of Allah (ﷺ) said: "Rejoice, 'Ammar, the transgressing party shall kill you."

حَدَّثَنَا أَبُو مُصْعَبٍ الْمَدَنِيُّ، قَالَ حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ مُحَمَّدٍ، عَنِ الْعَلاَءِ بْنِ عَبْدِ الرَّحْمَنِ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، رضى الله عنه قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ أَبْشِرْ عَمَّارُ تَقْتُلُكَ الْفِئَةُ الْبَاغِيَةُ ‏"‏ ‏. وَفِي الْبَابِ عَنْ أُمِّ سَلَمَةَ وَعَبْدِ اللَّهِ بْنِ عَمْرٍو وَأَبِي الْيَسَرِ وَحُذَيْفَةَ ‏. وَهَذَا حَدِيثٌ حَسَنٌ صَحِيحٌ غَرِيبٌ مِنْ حَدِيثِ الْعَلاَءِ بْنِ عَبْدِ الرَّحْمَنِ ‏.‏

Grade: Sahih (Darussalam)
Reference  : Jami` at-Tirmidhi 3800
In-book reference  : Book 49, Hadith 200
 
Why Imam Ali entered into negotiations with Mu'awiyah.
Quran 8:61: If the enemy incline towards peace, make peace with them.

Reference: Quran: Al-Anfal 8:61

Quran: Al-Anfal 8:61
Translation: Dr. Mustafa Khattab, the Clear Quran

If the enemy is inclined towards peace, make peace with them. And put your trust in Allah. Indeed, He ˹alone˺ is the All-Hearing, All-Knowing.

‏ وَإِن جَنَحُوا۟ لِلسَّلْمِ فَٱجْنَحْ لَهَا وَتَوَكَّلْ عَلَى ٱللَّهِ ۚ إِنَّهُۥ هُوَ ٱلسَّمِيعُ ٱلْعَلِيمُ

Reference  : Quran: Al-Anfal 8:61

"Enemy" is determined from the context of the verse, specifically Allah swt is talking about the enemies in the prior verse.

 
When questioned, the companions cited the peace that the Prophet brokered with the Meccans at Hudaybiyah. Peace is victory.

Reference: Sahih al-Bukhari 4844

Sahih al-Bukhari 4844
Narrated Habib bin Abi Thabit:

I went to Abu Wail to ask him (about those who had rebelled against `Ali). On that Abu Wail said, "We were at Siffin (a city on the bank of the Euphrates, the place where me battle took place between `Ali and Muawiya) A man said, "Will you be on the side of those who are called to consult Allah's Book (to settle the dispute)?" `Ali said, 'Yes (I agree that we should settle the matter in the light of the Qur'an)." ' Some people objected to `Ali's agreement and wanted to fight. On that Sahl bin Hunaif said, 'Blame yourselves! I remember how, on the day of Al-Hudaibiya (i.e. the peace treaty between the Prophet (ﷺ) and the Quraish pagans), if we had been allowed to choose fighting, we would have fought (the pagans). At that time `Umar came (to the Prophet) and said, "Aren't we on the right (path) and they (pagans) in the wrong? Won't our killed persons go to Paradise, and theirs in the Fire?" The Prophet replied, "Yes." `Umar further said, "Then why should we let our religion be degraded and return before Allah has settled the matter between us?" The Prophet (ﷺ) said, "O the son of Al-Khattab! No doubt, I am Allah's Messenger (ﷺ) and Allah will never neglect me." So `Umar left the place angrily and he was so impatient that he went to Abu Bakr and said, "O Abu Bakr! Aren't we on the right (path) and they (pagans) on the wrong?" Abu Bakr said, "O the son of Al-Khattab! He is Allah's Messenger (ﷺ), and Allah will never neglect him." Then Sura Al-Fath (The Victory) was revealed."

حَدَّثَنَا أَحْمَدُ بْنُ إِسْحَاقَ السُّلَمِيُّ، حَدَّثَنَا يَعْلَى، حَدَّثَنَا عَبْدُ الْعَزِيزِ بْنُ سِيَاهٍ، عَنْ حَبِيبِ بْنِ أَبِي ثَابِتٍ، قَالَ أَتَيْتُ أَبَا وَائِلٍ أَسْأَلُهُ فَقَالَ كُنَّا بِصِفِّينَ فَقَالَ رَجُلٌ أَلَمْ تَرَ إِلَى الَّذِينَ يُدْعَوْنَ إِلَى كِتَابِ اللَّهِ‏.‏ فَقَالَ عَلِيٌّ نَعَمْ‏.‏ فَقَالَ سَهْلُ بْنُ حُنَيْفٍ اتَّهِمُوا أَنْفُسَكُمْ فَلَقَدْ رَأَيْتُنَا يَوْمَ الْحُدَيْبِيَةِ ـ يَعْنِي الصُّلْحَ الَّذِي كَانَ بَيْنَ النَّبِيِّ صلى الله عليه وسلم وَالْمُشْرِكِينَ ـ وَلَوْ نَرَى قِتَالاً لَقَاتَلْنَا، فَجَاءَ عُمَرُ فَقَالَ أَلَسْنَا عَلَى الْحَقِّ وَهُمْ عَلَى الْبَاطِلِ أَلَيْسَ قَتْلاَنَا فِي الْجَنَّةِ وَقَتْلاَهُمْ فِي النَّارِ قَالَ ‏"‏ بَلَى ‏"‏‏.‏ قَالَ فَفِيمَ أُعْطِي الدَّنِيَّةَ فِي دِينِنَا، وَنَرْجِعُ وَلَمَّا يَحْكُمِ اللَّهُ بَيْنَنَا‏.‏ فَقَالَ ‏"‏ يَا ابْنَ الْخَطَّابِ إِنِّي رَسُولُ اللَّهِ وَلَنْ يُضَيِّعَنِي اللَّهُ أَبَدًا ‏"‏‏.‏ فَرَجَعَ مُتَغَيِّظًا، فَلَمْ يَصْبِرْ حَتَّى جَاءَ أَبَا بَكْرٍ فَقَالَ يَا أَبَا بَكْرٍ أَلَسْنَا عَلَى الْحَقِّ وَهُمْ عَلَى الْبَاطِلِ قَالَ يَا ابْنَ الْخَطَّابِ إِنَّهُ رَسُولُ اللَّهِ صلى الله عليه وسلم وَلَنْ يُضَيِّعَهُ اللَّهُ أَبَدًا‏.‏ فَنَزَلَتْ سُورَةُ الْفَتْحِ‏.‏

Reference  : Sahih al-Bukhari 4844
In-book reference  : Book 65, Hadith 365
 
Implications on Mu'awiyah.
Rasulullah (ﷺ) said: Killing a Muslim is Kufur.

Reference: Sahih al-Bukhari 6044

Sahih al-Bukhari 6044
Narrated `Abdullah:

Allah's Messenger (ﷺ) said, "Abusing a Muslim is Fusuq (i.e., an evil-doing), and killing him is Kufr (disbelief).

حَدَّثَنَا سُلَيْمَانُ بْنُ حَرْبٍ، حَدَّثَنَا شُعْبَةُ، عَنْ مَنْصُورٍ، قَالَ سَمِعْتُ أَبَا وَائِلٍ، يُحَدِّثُ عَنْ عَبْدِ اللَّهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ سِبَابُ الْمُسْلِمِ فُسُوقٌ، وَقِتَالُهُ كُفْرٌ ‏"‏‏.‏ تَابَعَهُ غُنْدَرٌ عَنْ شُعْبَةَ‏.‏
Reference  : Sahih al-Bukhari 6044
In-book reference  : Book 78, Hadith 74
 
Rasulullah (ﷺ) said: None but a believer would love Ali (RA) and none but a hypocrite would hate Ali (RA)

Reference: Sunan Ibn Majah 114

Sunan Ibn Majah 114
It was narrated that 'Ali said:

The Unlettered Prophet informed me (saying) that none but a believer would love me and none but a hypocrite would hate me

حَدَّثَنَا عَلِيُّ بْنُ مُحَمَّدٍ، حَدَّثَنَا وَكِيعٌ، وَأَبُو مُعَاوِيَةَ وَعَبْدُ اللَّهِ بْنُ نُمَيْرٍ عَنِ الأَعْمَشِ، عَنْ عَدِيِّ بْنِ ثَابِتٍ، عَنْ زِرِّ بْنِ حُبَيْشٍ، عَنْ عَلِيٍّ، قَالَ عَهِدَ إِلَىَّ النَّبِيُّ الأُمِّيُّ ـ صلى الله عليه وسلم ـ أَنَّهُ لاَ يُحِبُّنِي إِلاَّ مُؤْمِنٌ وَلاَ يُبْغِضُنِي إِلاَّ مُنَافِقٌ ‏.
Reference  : Sunan Ibn Majah 114
In-book reference  : Introduction, Hadith 114