In a two muslim's combat one muslim kills another, both will be doom to hell

Hadith of the day 

The Prophet (ﷺ) said, "When two Muslims are engaged in a combat against each other with their sword's and one is killed, both are doomed to Hell". I said, "O Messenger of Allah! As to the one who kills, it is understandable, but why the slain one?" He (ﷺ) replied: "He was eager to kill his opponent". 

[Bukhari et Muslim, riyad as-salihin 9] 

This hadith warns against the severe consequences of Muslims fighting each other with the intent to kill. The Prophet Muhammad (ﷺ) explains that when two Muslims engage in such combat, both are deserving of punishment in Hell, even if only one actually kills the other. When the question arises as to why the one who was slain is also punished, the Prophet (ﷺ) clarifies that the slain person had the same intention—to kill his opponent.

The emphasis here is on the intention behind the action. In Islam, the intention (niyyah) plays a crucial role in determining the morality of a deed. Even if a person does not succeed in their wrongful action, if their intention was sinful, they are still accountable for it. The hadith serves as a powerful reminder to avoid conflicts, violence, and harboring harmful intentions toward others, especially within the Muslim community. Islam promotes peace, and aggression, especially between believers, is strongly condemned.