Moral justification for the execution of criminals of war?

[Jehanne]

Death is annihilation, and so, an execution, once completed, is no longer punishment.  If you want to punish murderers, leave them in prison.

I get your point. But in the Nuremberg trials, the Nazis who commited the worse crimes were the ones executed. Rest were imprisoned. Ending someone's life is considered a greater punishment than imprisonment, at least socially.

That's the typical religious attitude -- "After death comes judgment."