All Muslims should read the Islamic history well so as to be able to answer such ignorant criticism.
The issue is of jewish tribe of Banu Quraizah who were allies of Muslim and had signed "meesaq-i-madinah". However, during the battle of trenches (Ghazwa-i-Khandaq), they changed sides and colluded with quraish against Muslims of Madinah.
In order to punish them for this treachery, Muslim forces laid siege to their fort. When Banu Quraizah could not fight anymore, they negotiated for surrender terms. As a condition of their surrender, they laid the condition that Prophet Muhammad (PBUH) would not decide their fate and instead this decision will be made by the head of their old allies Banu Aus (a branch of Ansaar of Madinah). The head of Banu Aus was Saad bin Muaaz.
Saad bin Muaaz said that he will decide the fate of vanquished Banu Quraizah according to the laws of jewsih holy book Torah. According to Torah (and as historically practiced by jewish kings) the men-folk of the vanquished tribe are killed and their properties confiscated. Accordingly the same Torah prescribed treatment was meted out to the jews of Banu Quraizah.
This killing of Banu Quraizah men was the decision of Saad bin Muaaz in light of jewish teachings who was chosen as arbiter of their fate by Banu Quraizah themselves.
If Banu Quraizah had accepted Prophet Muhammad (PBUH) as the arbiter, in likelihood the most punishment they could expect was exile as happened to the two other jewish tribes of Banu Nuzair and Banu Qainqa'a who had similarly conspired against Muslims.