Koran, 33:26-27. The incident ofthe Banu-Qurayzah is beset by known and unknown of the case is that they betrayed their and took advantage of difficult circumstances when the confederates  (the Ahzab) besieged al-Madinah. Muslim eyes went astray and their hearts lumped in their throats from fear . The Banu-Qurayzah then reneged on their pledge and stabbed the Muslims in the back .Another reason is that  they fell  under the jurisdiction of the chief of the Aws , Sa'd ibn-Ma'adh, since they were his associate, who meted out judgment to them accordingly.They surrendered on a condition,and the condition was their to observe.It is also said that the death sentences which they recievedwere in accordance with Jewish law (since they were a Jewish tribe),and that Sa'd judged them by their own laws. The episode as a whole has an air of mystery about it which leads us to believe there were aditional causes unknown to us.