I don't believe that the death penalty should exist

In the United States we believe in certain "unalienable rights." One of those is the right to life. Of course, this was within the Declaration of Independence, and has no force of binding law, but it's still a core idea of the country nonetheless.

If we, here, all believe in that right, the fact that someone has done something monstrous, or many monstrous things, cannot revoke that right, as it is "unalienable." An unnecessary taking of someone's life would be a violation of that right.

Now I say unnecessary very deliberately. If a person kills another in self-defense, that is an entirely necessary violation of the attacker's unalienable right to life. But after capture? Within prison? The violation is no longer immediately necessary.