This verse was revealed in Mecca:" And they who call not upon another god with Allah and slay not the soul which Allah has forbidden except in the cause of justice" up to the word Muhdana (abased). Thereupon the polytheists said: Islam is of no avail to us for we have made peer with Allah and we killed the soul which Allah had forbidden to do and we committed debauchery, and it was (on this occasion) that Allah, the Exalted and Glorious, revealed this verse:" Except him who repents and believes and does good deeds" up to the end Ibn 'Abbis says: He who enters the fold of Islam and understands its command and then kills the soul there is no repentance for him.  

عَبَّاسٍقَالَ نَزَلَتْ هَذِهِ الآيَةُ بِمَكَّةَ { وَالَّذِينَ لاَ يَدْعُونَ مَعَ اللَّهِ إِلَهًا آخَرَ} إِلَى قَوْلِهِ { مُهَانًا} فَقَالَ الْمُشْرِكُونَ وَمَا يُغْنِي عَنَّا الإِسْلاَمُ وَقَدْ عَدَلْنَا بِاللَّهِ وَقَدْ قَتَلْنَا النَّفْسَ الَّتِي حَرَّمَ اللَّهُ وَأَتَيْنَا الْفَوَاحِشَ فَأَنْزَلَ اللَّهُ ﷻ { إِلاَّ مَنْ تَابَ وَآمَنَ وَعَمِلَ عَمَلاً صَالِحًا} إِلَى آخِرِ الآيَةِ قَالَ فَأَمَّا مَنْ دَخَلَ فِي الإِسْلاَمِ وَعَقَلَهُ ثُمَّ قَتَلَ فَلاَ تَوْبَةَ لَهُ  

I asked Ibn 'Abbas (about the verse relating to intentional homicide in Surat An-Nisa') He said: When the verse "Those who invoke not with Allah any other god, nor slay such life as Allah had made sacred, except for just cause" was revealed, the polytheists of Mecca said: We have killed the soul prohibited by Allah, invoked another god along with Allah for worship, and committed shameful deeds. So Allah revealed the verse "unless he repents, believes, and works righteous deeds, for Allah will change the evil of such persons into good." This is meant for them. As regards the verse "if a man kills a believer intentionally, his recompense is Hell" He said: If a man knows the command of Islam and intentionally kills a believer, his repentance wil not be accepted. I then mentioned it to Mujahid. He said: "Except the one who is ashamed (of his sin)."  

لَمَّا نَزَلَتِ الَّتِي فِي الْفُرْقَانِ { وَالَّذِينَ لاَ يَدْعُونَ مَعَ اللَّهِ إِلَهًا آخَرَ وَلاَ يَقْتُلُونَ النَّفْسَ الَّتِي حَرَّمَ اللَّهُ إِلاَّ بِالْحَقِّ } قَالَ مُشْرِكُو أَهْلِ مَكَّةَ قَدْ قَتَلْنَا النَّفْسَ الَّتِي حَرَّمَ اللَّهُ وَدَعَوْنَا مَعَ اللَّهِ إِلَهًا آخَرَ وَأَتَيْنَا الْفَوَاحِشَ فَأَنْزَلَ اللَّهُ { إِلاَّ مَنْ تَابَ وَآمَنَ وَعَمِلَ عَمَلاً صَالِحًا فَأُولَئِكَ يُبَدِّلُ اللَّهُ سَيِّئَاتِهِمْ حَسَنَاتٍ } فَهَذِهِ لأُولَئِكَ قَالَ وَأَمَّا الَّتِي فِي النِّسَاءِ { وَمَنْ يَقْتُلْ مُؤْمِنًا مُتَعَمِّدًا فَجَزَاؤُهُ جَهَنَّمُ } الآيَةُ قَالَ الرَّجُلُ إِذَا عَرَفَ شَرَائِعَ الإِسْلاَمِ ثُمَّ قَتَلَ مُؤْمِنًا مُتَعَمِّدًا فَجَزَاؤُهُ جَهَنَّمُ لاَ تَوْبَةَ لَهُ فَذَكَرْتُ هَذَا لِمُجَاهِدٍ فَقَالَ إِلاَّ مَنْ نَدِمَ