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]