Abu Bakrah Ath-Thaqafi (May Allah be pleased with him) reported:
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".

[Al-Bukhari and Muslim].

وعن أبي بكرة نفيع بن الحارث الثقفى رضي الله عنه أن النبي ﷺ قال: "إذ التقى المسلمان بسيفيهما فالقاتل والمقتول في النار" قلت يارسول الله، هذا القاتل فما بال المقتول؟ قال: "إنه كان حريصاً على قتل صاحبه" ((متفق عليه)).