Personally, when I think about the death penalty, it isn't about considering what that person has done, it's more a matter of pragmatism. It's like when you've got a wild animal that's developed a taste for humans; you don't hunt it down an kill it to teach the animal a lesson, you hunt it down and kill it so that it can't kill again.
When you have a violent criminal who's violent because they've had a shitty life and make a living through criminal acts, or because they acted hastily in a bad situation, you lock that person away. There's a chance that person can be redeemed, so you separate them from society for a while and hope they're ready to clean up their act once they're released (not a very good system, but hey, you got any better ideas?)
When you have a violent criminal, like Charles Manson or John Wayne Gacy, who's violent because they are insane, or they just love hurting people more than anything else, then you kill them. It's not a matter of punishment; really, what's the point in punishing someone like that? Punishing that person isn't going to right any of the wrongs they committed. The people they've hurt won't get un-hurt by torturing the crazy bastards. The best thing you can do to those kinds of people is to get rid of them as quickly and efficiently as our criminal justice systems will allow, so that they can't hurt anyone else and all of us can rest a little easier at night.
So, referencing OT, when I think of the death penalty, asking if that person "deserves death" is the wrong question, because the death penalty isn't really a "punishment" per-se. You only punish something when you have some hope of correcting its behavior, so saying that death penalty is a punishment is probably fallacious. For people like Manson and Gacy, it's less accurate to say that they "deserved to die", and more accurate to say "we had no choice but to kill them". People who commit such heinous acts pose such a dire threat that our fear of them rightfully overrides any sympathy we might have for them, or any moral qualms we might have about taking their lives.
Bottom line: When someone "deserves death", it's not really a matter of them "deserving" anything. It's more a matter of "what is the most effective way to protect ourselves from this person". It's not about revenge or punishment, it's a rational, pragmatic decision made to protect society from its most dangerous members.