When critics of Islam point to the execution of the men of Banu Quraydhah as an example of the unique barbarism of the religion (and they always do this), there is a double irony.
The judgement of execution was made by Sa’d bin Mu’adh (not by the Prohpet), as he was the arbitrator of their choice; and the judgement was made in accordance with Jewish law, not the Shari’ah, which was, again, their choice.(Deuteronomy 20: 13-14)
So in fact, the critics are condemning the law of the Torah, not the Qur’an. They are ascribing to Islam a uniquely savage level of violence, while, in fact, it was a ruling that was not made by the Prophet of Islam, and which was only made in compliance with Jewish law, as per the wishes of Banu Quraydhah themselves.