The Prophet ﷺ said, "When two Muslims confront one another with their swords, each intending to kill the other, both of them are in the Fire."Someone asked, "Messenger of Allah, this is the killer, but what about the one who was killed?"He replied, "He too was eager to kill his companion."
أخبرنا علي بن محمد بن علي المصيصي قال حدثنا خلف عن زائدة عن هشام عن الحسن
عن النبي ﷺ قال إذا تواجه المسلمان بسيفيهما كل واحد منهما يريد قتل صاحبه فهما في النار قيل له يا رسول الله هذا القاتل فما بال المقتول قال إنه كان حريصا على قتل صاحبه
[Sahih] Ahmad narrated it: 46/5, 47, 51, through two routes from Hasan Basri, with this wording, and it is the greater one, H: 3585, and it has many corroborating narrations, see the following hadith numbered: 4127.