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.»  

While I was going to help this man ('Ali Ibn Abi Talib), Abu Bakra met me and asked, "Where are you going?" I replied, "I am going to help that person." He said, "Go back for I have heard Messenger of Allah ﷺ saying, 'When two Muslims fight (meet) each other with their swords, both the murderer as well as the murdered will go to the Hell-fire.' I said, 'O Messenger of Allah ﷺ! It is all right for the murderer but what about the murdered one?' Messenger of Allah ﷺ replied, "He surely had the intention to kill his companion."  

ذَهَبْتُ لأَنْصُرَ هَذَا الرَّجُلَ فَلَقِيَنِي أَبُو بَكْرَةَ، فَقَالَ: أَيْنَ تُرِيدُ؟ قُلْتُ: أَنْصُرُ هَذَا الرَّجُلَ۔ قَالَ: ارْجِعْ، فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ: «إِذَا الْتَقَى الْمُسْلِمَانِ بِسَيْفَيْهِمَا فَالْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ۔» فَقُلْتُ: يَا رَسُولَ اللَّهِ، هَذَا الْقَاتِلُ، فَمَا بَالُ الْمَقْتُولِ؟ قَالَ: «إِنَّهُ كَانَ حَرِيصًا عَلَى قَتْلِ صَاحِبِهِ۔»  

There was a person before you who had killed ninety-nine persons and then made an inquiry about the learned persons of the world (who could show him the way to salvation). He was directed to a monk. He came to him and told him that he had killed ninety-nine persons and asked him whether there was any scope for his repentance to be accepted. He said: No. He killed him also and thus completed one hundred. He then asked about the learned persons of the earth and he was directed to a scholar, and he told him that he had killed one hundred persons and asked him whether there was any scope for his repentance to be accepted. He said: Yes; what stands between you and repentance? You better go to such and such land; there are people devoted to prayer and worship and you also worship along with them and do not come to the land of yours since it was an evil land (for you).

So he went away and he had hardly covered half the distance when death came to him and there was a dispute between the angels of mercy and the angels of punishment. The angels of mercy said: This man has come as a penitent and remorseful to Allah and the angels of punishment said: He has done no good at all. Then there came another angel in the form of a human being in order to decide between them. He said: You measure the land to which he has drawn near. They measured it and found him nearer to the land where he intended to go (the land of piety), and so the angels of mercy took possession of it. Qatada said that Hasan told him that it was said to them that as death approached him, he crawled upon his chest (and managed) to slip in the land of mercy.  

أَنَّ نَبِيَّ اللَّهِ ﷺ قَالَ: «كَانَ فِيمَنْ كَانَ قَبْلَكُمْ رَجُلٌ قَتَلَ تِسْعَةً وَتِسْعِينَ نَفْسًا۔ فَسَأَلَ عَنْ أَعْلَمِ أَهْلِ الأَرْضِ، فَدُلَّ عَلَى رَاهِبٍ۔ فَأَتَاهُ فَقَالَ إِنَّهُ قَتَلَ تِسْعَةً وَتِسْعِينَ نَفْسًا فَهَلْ لَهُ مِنَ تَوْبَةٍ۔ فَقَالَ لاَ، فَقَتَلَهُ فَكَمَّلَ بِهِ مِائَةً۔ ثُمَّ سَأَلَ عَنْ أَعْلَمِ أَهْلِ الأَرْضِ، فَدُلَّ عَلَى رَجُلٍ عَالِمٍ۔ فَقَالَ إِنَّهُ قَتَلَ مِائَةَ نَفْسٍ فَهَلْ لَهُ مِنْ تَوْبَةٍ۔ فَقَالَ نَعَمْ وَمَنْ يَحُولُ بَيْنَهُ وَبَيْنَ التَّوْبَةِ؟ انْطَلِقْ إِلَى أَرْضِ كَذَا وَكَذَا فَإِنَّ بِهَا أُنَاسًا يَعْبُدُونَ اللَّهَ، فَاعْبُدِ اللَّهَ مَعَهُمْ وَلاَ تَرْجِعْ إِلَى أَرْضِكَ فَإِنَّهَا أَرْضُ سَوْءٍ۔

فَانْطَلَقَ حَتَّى إِذَا نَصَفَ الطَّرِيقَ أَتَاهُ الْمَوْتُ، فَاخْتَصَمَتْ فِيهِ مَلاَئِكَةُ الرَّحْمَةِ وَمَلاَئِكَةُ الْعَذَابِ۔ فَقَالَتْ مَلاَئِكَةُ الرَّحْمَةِ: جَاءَ تَائِبًا مُقْبِلاً بِقَلْبِهِ إِلَى اللَّهِ۔ وَقَالَتْ مَلاَئِكَةُ الْعَذَابِ: إِنَّهُ لَمْ يَعْمَلْ خَيْرًا قَطُّ۔ فَأَتَاهُمْ مَلَكٌ فِي صُورَةِ آدَمِيٍّ فَجَعَلُوهُ بَيْنَهُمْ۔ فَقَالَ: قِيسُوا مَا بَيْنَ الأَرْضَيْنِ فَإِلَى أَيَّتِهِمَا كَانَ أَدْنَى فَهُوَ لَهُ۔ فَقَاسُوهُ فَوَجَدُوهُ أَدْنَى إِلَى الأَرْضِ الَّتِي أَرَادَ، فَقَبَضَتْهُ مَلاَئِكَةُ الرَّحْمَةِ۔

قَالَ قَتَادَةُ فَقَالَ الْحَسَنُ ذُكِرَ لَنَا أَنَّهُ لَمَّا أَتَاهُ الْمَوْتُ نَأَى بِصَدْرِهِ