Abu Bakrah Nufaī‘ ibn al-Hārith Ath-Thaqafi (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "When two Muslims meet to fight each other with their swords, both the killer and the killed are doomed to Hell." I said: "O Messenger of Allah, that is the case with the killer, but what about the one killed?" He replied: "Indeed, he was keen to kill his companion."
شرح الحديث :
If two Muslims meet each other with their swords each intending to keel the other one, then the killer - because of his killing his fellow, and the killed because he was also eager to fight and kill the other one - both of them are doomed to Hellfire, if Allah does not pardon them.