Allah, Glorified and Exalted, laughs at two men, one of them kills the other, yet both enter Paradise. This one fights in the cause of Allah, so he gets killed. Then Allah accepts the repentance of the killer, so he becomes a Muslim and dies as a martyr
Authentic hadithعن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: «يضحكُ اللهُ سبحانه وتعالى إلى رجلين يقتلُ أحدُهما الآخرَ يدخلانِ الجنةَ، يقاتلُ هذا في سبيل الله فيُقْتَلُ، ثم يتوبُ اللهُ على القاتلِ فيُسْلِمُ فَيُسْتَشْهَدُ».
Abu Hurayrah (may Allah be pleased with him) narrated directly from the Prophet (may Allah's peace and blessings be upon him) that he said: "Allah, Glorified and Exalted, laughs at two men, one of them kills the other, yet both enter Paradise. This one fights in the cause of Allah, so he gets killed. Then Allah forgives the killer, and becomes a Muslim and dies as a martyr."
Explanation
This Hadīth indicates that Allah laughs at two men. Although one of them kills the other, both will enter Paradise. A Muslim fights in the cause of Allah to elevate the word of Allah, yet he gets killed by a disbeliever, so he enters Paradise. Later Allah accepts the repentance of the killer, so he accepts Islam. Then he fights in the cause of Allah and dies as a martyr.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).