Do people really want to hand the state the right to commit murder? Really? Because that's what the death penalty is...state sanctioned murder.
Is murder wrong or isn't it? If you're in favour of the death penalty then you must believe that it's right if the circumstances are right. And who decides that? A jury? Given how often juries get things wrong, I wouldn't trust one with anyone's life, let alone my own.
In any event, the death penalty achieves nothing.
It doesn't prevent crime. Most crimes attracting the death penalty are not pre-planned so the deterrent aspect wouldn't have any effect, anyway.
We also know that the states with the death penalty have a higher crime rate than those without - probably due to the inherent, underlying violence of a government with the right to murder you.
So, as a deterrant, it's useless.
Revenge? Justice is not about revenge. If it was, rapists would be raped & burglars would be burgled. It's only the crime of murder where the whole "eye for an eye" thing gets trotted out, which really makes no sense.
Bottom line, you can't retain moral authority by telling people that murder is so wrong, we'll murder you if you do it.
And this doesn't even touch on the issue of possible innocence. Even with advances in DNA, it's impossible to know with 100% certainty that someone is guilty. Labs are run by human beings and human beings occasionally make mistakes. Killing an innocent is a crime so devastating and shocking that we should do everything we can to avoid it ever happening - first step, not kill them in the first place.
"If anyone hurt my DC, I'd want them dead" is not a good argument. We all would. The emotional pain from that kind of loss would be overrwhelming. But the law is there to be objective when we can't.
I think the only justification for killing a human being is to prevent them killing either yourself or others. There's no other justification, imo.