To me the death penalty is a bad solution to a shitty problem. Before anyone climbs down my fucking throat, it's not black and white, there will always be those moments where you can't just say "no, never kill" - life would be so goddamned easy if anything worked like that. As a standard punishment it's just a bad use of resources and will inevitably have its share of innocent victims.
I've heard it numerous times; "What if you knew without a shadow of a doubt that the person was guilty?" and the answer from me will always be: "But I don't." Even one innocent victim of death penalty is bad collateral. It's not a question of whether the person deserves death, it's whether we deserve to be forced to kill a person as long as we have other options. You wouldn't trust your government to handle school lunches, let alone whether someone should die.
As for the "it's expensive to keep them alive" yeah, it is, because the courts and bureaucracy are fucking wasteful. I still wouldn't support the death penalty even if someone just volunteered to cap their ass with a shotgun. I also wouldn't fight the convictions of serial killers, rapist-slash-murderers who took down entire families like it was nothing being strapped to the chair. It should always be an absolute extreme option to an extreme problem with no repentance. If they want to find Jesus in their cells while they spend the rest of their lives in there, that's fine. If they're any threat to the prison staff or other prisoners, no mercy. As much as I hate the death penalty, I'm not looking to risk anyone's life for the sake of a principle.
But euthanasia, that I do encourage. A prisoner with a life sentence should have the freedom to choose death. Clean and easy, just write your name down and sit on the chair, no long-winded court appeals. Hell, just hand them a rope and say good night.