The details of the Banu Qurayza event where 600, 800 or 900 men were put to death are questionnable.
Ibn Ishaq (151 AH) is the earliest historian to have recorded this event, 145 years after it happened. Ibn Ishaq is the longest, most detailed and quoted text.
Historians and biographers were like 'data collectors'. Their role was to collate as much information as possible, and not to verify information authenticity or reliability. Ibn Ishaq was no different!
To kill such a large number of people is diametrically opposed to the Islamic sense of justice and to basic principles in the Quran
It is unlikely Banu Qurayza tribe was slaughtered when the other Jewish groups who surrendered before, and after them were treated leniently and allowed to go free
Later, Muslim scholars and historians view Ibn Ishaqs version with complacency, uncertainty, condemnatlon and even outright rejection.
In conclusion, the Banu Qurayza event was a hostile war situation where men would have died. However, it is factually incorrect to claim that 800 men were slaughtered as the evidence for that is not their.
For a more detailed answer, see haqq.com.au/~salam/misc/qurayza.html