Moral justification for the execution of criminals of war?

[Deesse23]

Please re-read what you wrote here, and let it sink in for 5 seconds.

I'd say that a life sentence is pretty much the end of a meaningful human existence.

A lot of people, including people who are serving a life sentence, disagree with you.
A lot of people who found a meaningful life behind bars demonstrate you being wrong with this generalisation.
Of course your life would be meaningful behind bars, maybe under any circumstance, i dont deny that.