Hazrat Ammar Yasir is one of the great Sahaba who stood by side of Imam Ali asws
and was Martyred by rebel party led by Mawia when they fought him in battle of Siffen

Let us see what Holy Prophet asws had said for him

رقم الحديث: 7238
(حديث مرفوع) أَخْبَرَنَا عِمْرَانُ بْنُ مُوسَى بْنِ مُجَاشِعٍ ، حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ ، حَدَّثَنَا يَزِيدُ بْنُ هَارُونَ ، حَدَّثَنَا الْعَوَّامُ بْنُ حَوْشَبٍ ، عَنْ سَلَمَةَ بْنِ كُهَيْلٍ ، عَنْ عَلْقَمَةَ ، عَنْ خَالِدِ بْنِ الْوَلِيدِ ، قَالَ : كَانَ بَيْنِي وَبَيْنَ عَمَّارِ بْنِ يَاسِرٍ كَلامٌ ، فَانْطَلَقَ عَمَّارٌ يَشْكُوا إِِلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ، قَالَ : فَجَعَلَ خَالِدٌ لا يَزِيدُهُ إِِلا غِلْظَةً وَرَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ سَاكِتٌ ، قَالَ : فَبَكَى عَمَّارٌ ، وَقَالَ : يَا رَسُولَ اللَّهِ ، أَلا تَسْمَعُهُ ؟ قَالَ : فَرَفَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِِلَيَّ رَأْسَهُ ، وَقَالَ : ” مَنْ عَادَى عَمَّارًا عَادَاهُ اللَّهُ ، وَمَنْ أَبْغَضَهُ أَبْغَضَهُ اللَّهُ ” ، قَالَ : فَخَرَجْتُ ، فَمَا كَانَ شَيْءٌ أَحَبَّ إِِلَيَّ مِنْ رِضَا عَمَّارٍ ، فَلَقِيتُهُ فَرَضِيَ .

Khalid bin walid said that once I had a hard talk with Ammar, so he went to Holy Prophet asws and complained about it. Khalid also went there and when he reached there, voice of Ammar raised. Holy Prophet asws was standing silent. Ammar started crying, and said: O Propeht ! are you listening? Holy Prophet asws raised his head and said: Whosoever will have enemity for Ammar, he has enemity with Allah and whosoever will have grudge for him, he has grudge for Allah. Khalid said that I went out and at that time, nothing was more precious for me than making him happy. So I met him and made him happy

[Sahih Ibn Haban, vol 15, page 556-557]

Sheikh Shoaib ul arnawut says

Chain is Sahih on terms of Sheikhain

This narration is also present in urdu version of Musnad Ahmad, vol 7, page 13. And with some minor differences, it comes in Sair A’laam Nabala, vol 1, page 415; Sheikh Shoaib ul Arnawut has termed the chains Reliable there

Logically speaking! what will be the status of those who went out to the extent to Fight him and then kill him????