Atheists, the death penalty and abortion...

[Mister Agenda]

Because we punish someone so they will modify their behavior. Dead people can't modifiy their behavior.

You're begging the question. Why should we seek to modify, why not execute them?

The question was 'Why can't the death penalty be a punishment?'. I fail to see how I did not answer it, or in what way it was question-begging.

Perhaps you should consider that your real question might be 'Why can't the death penalty be a deterrant'; to which I would say, although it doesn't seem to be statistically, there's no reason, in principle, that it could not be. Perhaps if it was swift and certtain enough, it would be an effective deterrant.