Because trying to kill a man who’s not trying to kill you — but who is commanding hundreds of thousands of men trying to kill you — is unconscionable. Muses Volokh:

[W]here exactly is the connection to the death penalty? Consider the chief arguments against the death penalty: the person being executed might be innocent; it’s just wrong for the state to kill people; others can be kept equally safe by locking the person up for life; the death penalty is likely to be applied in arbitrary or prejudiced ways. None of them work here.

Like the man says: Pay attention, social cons.