How can I convince non muslims esp jews that killing of males of Banu Quraiza was not unfair/cruel?

Try explaining to me how any killing is fair and loving?

I just did some, admittedly quick, research on this incident. There seems to be some questions about the historical facts here. This incident occurred during the rise of Mohammad as the prophet of Islam. What seems to be agreed is that there were three tribes of Judaism living in the area where Mohammad was developing a following for Islam. That these tribes objected to the rise of Islam. That two of the tribes were conquered and allowed to leave the area, but that the 3rd tribe (Banu Quraiza), when conquered, had all there men killed and their women an children made slaves. (Note: this is a very brief and simplified summary).

There seems to be some question about the numbers involved and whether all adult males were put to death.

In my opinion, this matters little. What matters is that Islam appears to have condoned and glorified not only the wholesale slaughter of a conquered people, but the enslavement and abuse of the conquered people. One article I read seems to attribute justification for jihad to this incident.

It also seems to me that there is too much in Islam that can be perverted by those seeking to justify acts like 9/11. This is not saying that Islam itself is evil, but that it can be perverted by those seeking to justify their evil. Nor is Islam only in this. More evil has been done in the name of religion that for the sake of evil alone. Most religions have skeletons in their closets like this.

So to answer your question my friend, I don't believe you can. The killing and rape of a conquered people is unjustified by any standard, In my opinion. Your response should be to point out the good in Islam that offsets the evils of incidents like Banu Quraiza.