“That was a nation which has passed on. It will have what it earned, and you will have what you have earned. And you will not be asked about what they used to do.” (Qur’an 2:133-134)
If you would like to know more you are encouraged to read:
Are all the companions just?
What really happened at the battle of Siffin
Ali Ibn Abi Talib his ijtihad and burning people alive