Can we find something about this in Islam? Did Mohammed do that himself for example?
Let us examine what we found in Sirat Ibn Hisham V3 page 93, Death of al Nadr and Uqba.
As we see hier el Nadr was killed by Ali Bin abi Talib after he surrendered. And then Mohammed in a place called al safra killed Uqba. And just before being executed by Mohammed Uqba begged him saying where will my kids go without me Mohammed. Mohammed’s answer was Hell. And then he decapitated him.
Arabic Text.
قال ابن إسحاق حتى إذا كان رسول الله صلى الله عليه وسلم بالصفراء قتل النضر بن الحارث قتله علي بن أبي طالب كما أخبرني بعض أهل العلم من أهل مكة ثم خرج حتى إذا كان بعرق الظبية قتل عقبة ابن أبي معيط.
قال ابن إسحاق فقال عقبة حين أمر رسول الله صلى الله عليه وسلم بقتله فمن للصبية يا محمد قال النار وكان الذي قتله عاصم بن ثابت بن أبي الأقلح أخو بني عمرو بن عوف كما حدثني أبو عبيدة بن محمد بن عمار بن ياسر..
Further to make our resolution stronger, lets seek more actions like that in Quran to make sure it isn’t only Mohammed or his followers acting like that but actually it’s their god Allah who is encouraging it!
Further evidence when we read surat el Anfal vers 67,
It is not for any prophet to have captives until he hath made slaughter in the land. Ye desire the lure of this world and Allah desireth (for you) the Hereafter, and Allah is Mighty, Wise.
مَا كَانَ لِنَبِىٍّ أَن يَكُونَ لَهُ ۥۤ أَسۡرَىٰ حَتَّىٰ يُثۡخِنَ فِى ٱلۡأَرۡضِۚ تُرِيدُونَ عَرَضَ ٱلدُّنۡيَا وَٱللَّهُ يُرِيدُ ٱلۡأَخِرَةَۗ وَٱللَّهُ عَزِيزٌ حَكِيمٌ۬
We see that Allah is telling Mohammed that he didn’t do good by letting the prisoners of war live. He should kill them. Resources: tafsir el Tabbari, tafsir Ibn Kathir for surat el Anfal.
Tafsir al Tabari: delivered by Said Bin Gubair. It was not suitable or appropriate for a prophet to keep prisoners. That is why Allah told his prophet that the prisoners must die, and not be exchanged