Mukhtasar Bukhari (Abridged) 1220 — If someone killed Muslims

Narrated from Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah will smile at two men, one of whom killed the other, yet both will enter Paradise. One fought in the way of Allah and was martyred. Then Allah granted the killer the ability to repent (he became a Muslim), and he too was martyred (in the way of Allah)."