i can identify u from your questions, but i will not deny an answer for the sake of others.
Banu quraiza conspirated against muslims, incited pagans to raid on makka, while having treaty with Prophrt Mohammad PBUH. the treaty obliged them to not support makkans, but they broke the treaty.
They participated in Battle of trenches along makkan non believers. with ALLAH Help, Makkan had to give up besirging Madina. Once they fled, leaving their ally Banu Quraiza behind, Mohammad PBUH raided banu quraiza, besieged and forced tthem to suurender.
Banu quraiza were allowed to appoint the Judge who shall give the verdict in their case. which conquerer will give this liberty??? They chose Ibn Salam, expecting favour from him...It was Ibn Salam, an ex jew, who Ordered that All War age men shall be killed, because This was the Punishment for Breaching a treaty and Waging war.
I hope this answer should be sufficient to clear any doubt.
