just some more to take note is...the decision was made by sa'd bin muadh who the jews themselves chose to make the decision (they were even given the decision!):
The Siege of Bani-Kureizha
Now came the affair of the Bani- Kureizha. The victory in the opinion of the Muslims was hardly achieved as long as the Bani-Kureizha remained so near, and in such dangerous proximity to Medina. They had proved themselves traitors in spite of their sworn alliance, and had at one time almost surprised medina from their side, an even which, if successful, would have involved the general massacre of the Muslims. The Jews were therefore besieged and compelled to surrender in discretion.
The condition for Surrender put forward by Bani-Kureizha
The Jews made only one condition, that their punishment should be left to the judgment of the Ausite (Of the tribe of Aus) chief, Sa'd Ibn Muaz. This man, a fierce soldier who had been wounded in the attack, and indeed died from his wounds the next day, infuriated by their treacherous conduct, gave the sentence that the "fighting men" should be put to death, and that the women and children should become the slaves of the Muslims ; and this sentence was carried into execution.
"It was a harsh, bloody sentence, " says Lane-Poole, "worthy of the Episcopal generals of the army against the Albigenses, or of the deeds of the Augustan age of Puritanism ; but it must be remembered that the crime of these men was high treason against the State during a time of siege ; and those who have read how Wellington's march could be traced by bodies of deserters and pillagers hanging from the trees , need not be surprised at the summary execution of a traitorous clan."
These traitorous Jews brought their fate upon themselves. If they had been put to death, even without the judgment of Sa'd, it would have been in consonance with the principles which then prevailed. But they themselves had chosen Sa'd as the sole arbiter and judge of their fate ; they knew that his judgment was not at all contrary to the received notions and accordingly never murdered. They knew that if they had succeeded they would have massacred their enemies without compunction.
People judge the massacres of King David according to the "lights of his time". Even the fearful slaughters committed by the Christians in primitive times are judge according to the certain "lights". Why should not the defensive wars of the early Muslims be looked at from the same standpoint? But whatever the point of view, an unprejudiced mind will perceive that no blame can possibly attach to the Prophet in the execution of the Bani-Kureizha.