A man said, "O Messenger of Allah ﷺ! Which sin is the greatest in Allah's Sight?" The Prophet ﷺ said, "To set up a rival unto Allah though He Alone created you . " The man said, "What is next?" The Prophet ﷺ said, "To kill your son lest he should share your food with you." The man said, "What is next?" The Prophet said, "To commit illegal sexual intercourse with the wife of your neighbor." So Allah revealed in confirmation of this narration:-- 'And those who invoke not with Allah, any other god. Nor kill, such life as Allah has forbidden except for just cause nor commit illegal sexual intercourse. And whoever does this shall receive the punishment.' (25.68)  

رَجُلٌ يَا رَسُولَ اللَّهِ أَىُّ الذَّنْبِ أَكْبَرُ عِنْدَ اللَّهِ قَالَ أَنْ تَدْعُوَ لِلَّهِ نِدًّا وَهْوَ خَلَقَكَ قَالَ ثُمَّ أَىٌّ قَالَ ثُمَّ أَنْ تَقْتُلَ وَلَدَكَ أَنْ يَطْعَمَ مَعَكَ قَالَ ثُمَّ أَىٌّ قَالَ ثُمَّ أَنْ تُزَانِيَ بِحَلِيلَةِ جَارِكَ فَأَنْزَلَ اللَّهُ ﷻ تَصْدِيقَهَا {وَالَّذِينَ لاَ يَدْعُونَ مَعَ اللَّهِ إِلَهًا آخَرَ وَلاَ يَقْتُلُونَ النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ وَلاَ يَزْنُونَ وَمَنْ يَفْعَلْ ذَلِكَ يَلْقَ أَثَامًا} الآيَةَ  

Messenger of Allah ﷺ said, "A faithful believer remains at liberty regarding his religion unless he kills somebody unlawfully."  

قَالَ رَسُولُ اللَّهِ ﷺ لَنْ يَزَالَ الْمُؤْمِنُ فِي فُسْحَةٍ مِنْ دِينِهِ مَا لَمْ يُصِبْ دَمًا حَرَامًا  

One of the evil deeds with bad consequence from which there is no escape for the one who is involved in it is to kill someone unlawfully.  

إِنَّ مِنْ وَرْطَاتِ الأُمُورِ الَّتِي لاَ مَخْرَجَ لِمَنْ أَوْقَعَ نَفْسَهُ فِيهَا سَفْكَ الدَّمِ الْحَرَامِ بِغَيْرِ حِلِّهِ  

The Prophet ﷺ said, "The first cases to be decided among the people (on the Day of Resurrection) will be those of blood-shed."  

قَالَ النَّبِيُّ ﷺ أَوَّلُ مَا يُقْضَى بَيْنَ النَّاسِ فِي الدِّمَاءِ  

An ally of Bani Zuhra who took part in the battle of Badr with the Prophet, that he said, "O Allah's Apostle! If I meet an unbeliever and we have a fight, and he strikes my hand with the sword and cuts it off, and then takes refuge from me under a tree, and says, 'I have surrendered to Allah (i.e. embraced Islam),' may I kill him after he has said so?" Messenger of Allah ﷺ said, "Do not kill him." Al- Miqdad said, "But O Messenger of Allah ﷺ! He had chopped off one of my hands and he said that after he had cut it off. May I kill him?" The Prophet ﷺ said. "Do not kill him for if you kill him, he would be in the position in which you had been before you kill him, and you would be in the position in which he was before he said the sentence." The Prophet ﷺ also said to Al-Miqdad, "If a faithful believer conceals his faith (Islam) from the disbelievers, and then when he declares his Islam, you kill him, (you will be sinful). Remember that you were also concealing your faith (Islam) at Mecca before."  

وَكَانَ شَهِدَ بَدْرًا مَعَ النَّبِيِّ ﷺ أَنَّهُ قَالَ يَا رَسُولَ اللَّهِ إِنْ لَقِيتُ كَافِرًا فَاقْتَتَلْنَا فَضَرَبَ يَدِي بِالسَّيْفِ فَقَطَعَهَا ثُمَّ لاَذَ بِشَجَرَةٍ وَقَالَ أَسْلَمْتُ لِلَّهِ آقْتُلُهُ بَعْدَ أَنْ قَالَهَا قَالَ رَسُولُ اللَّهِ ﷺ لاَ تَقْتُلْهُ قَالَ يَا رَسُولَ اللَّهِ فَإِنَّهُ طَرَحَ إِحْدَى يَدَىَّ ثُمَّ قَالَ بَعْدَ مَا قَطَعَهَا آقْتُلُهُ قَالَ لاَ تَقْتُلْهُ فَإِنْ قَتَلْتَهُ فَإِنَّهُ بِمَنْزِلَتِكَ قَبْلَ أَنْ تَقْتُلَهُ وَأَنْتَ بِمَنْزِلَتِهِ قَبْلَ أَنْ يَقُولَ كَلِمَتَهُ الَّتِي قَالَ وَقَالَ حَبِيبُ بْنُ أَبِي عَمْرَةَ عَنْ سَعِيدٍ عَنِ ابْنِ عَبَّاسٍ قَالَ قَالَ النَّبِيُّ ﷺ لِلْمِقْدَادِ إِذَا كَانَ رَجُلٌ مُؤْمِنٌ يُخْفِي إِيمَانَهُ مَعَ قَوْمٍ كُفَّارٍ فَأَظْهَرَ إِيمَانَهُ فَقَتَلْتَهُ فَكَذَلِكَ كُنْتَ أَنْتَ تُخْفِي إِيمَانَكَ بِمَكَّةَ مِنْ قَبْلُ