God even accepts the repentance of a disbelieving murderer

Messenger of Allah ﷺ said, "Allah welcomes two men with a smile; one of whom kills the other and both of them enter Paradise. One fights in Allah's Cause and gets killed. Later on Allah forgives the 'killer who also get martyred (In Allah's Cause)."  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «يَضْحَكُ اللَّهُ إِلَى رَجُلَيْنِ يَقْتُلُ أَحَدُهُمَا الآخَرَ يَدْخُلاَنِ الْجَنَّةَ: يُقَاتِلُ هَذَا فِي سَبِيلِ اللَّهِ فَيُقْتَلُ، ثُمَّ يَتُوبُ اللَّهُ عَلَى الْقَاتِلِ فَيُسْتَشْهَدُ.»  

I went to Messenger of Allah ﷺ while he was at Khaibar after it had fallen in the Muslims' hands. I said, "O Messenger of Allah ﷺ! Give me a share (from the land of Khaibar)." One of the sons of Sa'id bin Al-'As said, "O Messenger of Allah ﷺ! Do not give him a share." I said, "This is the murderer of Ibn Qauqal." The son of Said bin Al-As said, "Strange! A Wabr (i.e. guinea pig) who has come down to us from the mountain of Qaduim (i.e. grazing place of sheep) blames me for killing a Muslim who was given superiority by Allah because of me, and Allah did not disgrace me at his hands (i.e. was not killed as an infidel)." (The sub-narrator said "I do not know whether the Prophet ﷺ gave him a share or not.")  

أَتَيْتُ رَسُولَ اللَّهِ ﷺ وَهْوَ بِخَيْبَرَ بَعْدَ مَا افْتَتَحُوهَا فَقُلْتُ يَا رَسُولَ اللَّهِ أَسْهِمْ لِي فَقَالَ بَعْضُ بَنِي سَعِيدِ بْنِ الْعَاصِ لاَ تُسْهِمْ لَهُ يَا رَسُولَ اللَّهِ فَقَالَ أَبُو هُرَيْرَةَ هَذَا قَاتِلُ ابْنِ قَوْقَلٍ فَقَالَ ابْنُ سَعِيدِ بْنِ الْعَاصِ وَاعَجَبًا لِوَبْرٍ تَدَلَّى عَلَيْنَا مِنْ قَدُومِ ضَأْنٍ يَنْعَى عَلَىَّ قَتْلَ رَجُلٍ مُسْلِمٍ أَكْرَمَهُ اللَّهُ عَلَى يَدَىَّ وَلَمْ يُهِنِّي عَلَى يَدَيْهِ قَالَ فَلاَ أَدْرِي أَسْهَمَ لَهُ أَمْ لَمْ يُسْهِمْ لَهُ قَالَ سُفْيَانُ وَحَدَّثَنِيهِ السَّعِيدِيُّ عَنْ جَدِّهِ عَنْ أَبِي هُرَيْرَةَ قَالَ أَبُو عَبْدِ اللَّهِ السَّعِيدِيُّ عَمْرُو بْنُ يَحْيَى بْنِ سَعِيدِ بْنِ عَمْرِو بْنِ سَعِيدِ بْنِ الْعَاصِ