There was no casualty in Banu Qurayza siege as well. But entire tribe was slaughtered. And remember that Banu Qurayza was one of 3 tribes that gave refuge to Muhammad and his followers. Within 2 years of the arrival of Muhammad, 2 of the 3 tribes on whose hospitality Muhammad was totally dependent on when he arrived, were driven out of Madina by Muhammad.
As soon as Muhammad gained power in Madina he confiscated land property and women of the tribes that hosted him and finally drove them out of Madina. This history is in prophets biography and hadiths, they will never teach you this in your Islamiat classes.
Rasulallah was not a just human being. When he captured Quraish he usually forgave them but has no mercy for tribes that were not related to him.
Even in battle of Badr, Rasulallah spared his uncles and his family members. Only 2 men were executed even though their families offered to pay more than the double ransom than paid for his own uncles. Why because those men were not related to Rasulallah or to Quraish.