The dead leadership can't change their decisions anymore. And the new leadership has no reason to assume that considering a peace deal will keep them alive. The US has already shown that they are happy to break the deal, then a couple years alter kill you anyways. Staying the course at least keeps the internal threats down (which are just as capable of killing any autocrat)

Threatening autocrats might work, but just bolt out of the blue decapitation strikes undermine future threats because they figure they'll get no warning. If you are threatening, you are bluffing and when you aren't bluffing, there are no threats.

Some religions create a strong incentive for adherent to die in certain ways (X number of virgins when you get into heaven, that sort of stuff).

But anyway, once they are dead, your option to target them is gone.