I read on a website that Ali ibn Abu Talib (may Allah be pleased with him) burnt some of the Kharitjites during his caliphate. But this made me confused due to the hadith we know where the Prophet (peace and blessings be upon him) forbad torturing others with fire since this is a sort of associating others with Allah. So how did Ali (may Allah be pleased with him) do this?

And upon you is the peace of Allah, together with his mercy and blessings.
This report was narrated by al-Bukhary (6922) on the authority of `Ikrimah who said: Heretics were brought before Ali (may Allah be pleased with him) and he burnt them. When Ibn `Abbas (may Allah be pleased with him) was informed about this, he said, “If I were in his place, I would not have burnt them for the Messenger of Allah (peace and blessings be upon him) forbad this saying, “Do not torment with the torment of Allah” and I would have killed them, for the Messenger of Allah (peace and blessings be upon him) said, “Whoever changes his religion, kill him.”” When a deviant group called al-Saba’iyyah, who were the followers of the Jewish `Abdullah ibn Saba’, went astray and believed that Ali (may Allah be pleased with him) was a god – we seek refuge with Allah from this – he (Ali) set them on fire and said, “When I saw such an enormous evil, I set them on fire and called.” Besides, this issue is a particular case that has no general application, as al-Shatiby said.
In general, there are many interpretations concerning this report, whether he burnt them after he had killed them, or he was just about to burn them but he did not. Whatever the case was, this was an opinion viewed by a companion that has nothing to do with associating gods with Allah. Burning a person is not permissible in the Shari`ah; but this does not amount to associating others with Allah. Associating others with Allah means to worship another god with Allah or to believe in other gods with Almighty Allah. Yes, the Prophet (peace and blessings be upon him) forbad burning others and said “None should torment with fire except Allah.” [Reported by al-Bukhary (3016)]
Yet, this does not mean that whoever burns others with fire is considered as associating others with Allah. It rather means that this punishment is a punishment in the Hereafter, not in this world. This is what we should believe. The issue has no relation to associating others with Allah. As mentioned above, this interpretation may prove untrue. Perhaps he intended to burn them but he did not or he intended to burn them after killing them. Even if he actually burnt them, this would be a kind of ijtihad from a companion that disagrees with the text. The ultimate reference is always to the text. Nonetheless, we have to believe that they acted according to their ijtihad and that they are illuminating guides.