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)." (Using translation from Bukhārī 2826)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ «يَضْحَكُ اللهُ إِلَى رَجُلَيْنِ يَقْتُلُ أَحَدُهُمَا الْآخَرَ كِلَاهُمَا يَدْخُلُ الْجَنَّةَ يُقَاتِلُ هَذَا فِي سَبِيلِ اللهِ فَيُقْتَلُ ثُمَّ يَتُوبُ اللهُ عَلَى الْقَاتِلِ فَيُقَاتِلُ فَيُسْتَشْهَدُ»  

"Two will never be gathered together in the Fire: A Muslim who killed a disbeliever then tried his best and did not deviate. And two will never be gathered together in the lungs of a believer: Dust in the cause of Allah, and the odor of Hell. And two will never be gathered in the heart of a salve: Faith and envy." (Using translation from Nasāʾī 3109)  

أَنَّ رَسُولَ اللهِ ﷺ قَالَ لَا يَجْتَمِعَانِ فِي النَّارِ مُسْلِمٌ قَتَلَ كَافِرًا ثُمَّ سَدَّدَ وَقَارَبَ وَلَا يَجْتَمِعَانِ فِي جَوْفِ مُؤْمِنٍ غُبَارٌ فِي سَبِيلِ اللهِ وَفَيْحُ جَهَنَّمَ وَلَا يَجْتَمِعَانِ فِي عَبْدٍ الْإِيمَانُ وَالْحَسَدُ