It doesn't, though. There's no evidence that the death penalty serves any sort of deterrent. It isn't a punitive measure, it's vengeance. If you're ok with vengeance I respect your opinion though I disagree, but we can't pretend it's anything but vengeance.
I believe the use of the death penalty in severe cases isn't driven by vengeance; it's more about preventing the individual from causing harm to others in the future. It's important to recognize that not all criminals can be successfully rehabilitated, such as psychopaths or serial killers.
However, if the death penalty were to exist, I believe it should be carried out by the person who pronounces the verdict personally. This would ensure that such a grave decision is not made lightly.
Could you not prevent them from causing additional harm by not killing them? What if you're wrong? There's plenty of reason to leave them alive, but only one real reason to kill them, and that reason is vengeance.
Are you familiar with how the psychopathic mind functions? They lack empathy, and I don't mean it in a pejorative sense, no, I mean it literally.
Psychopaths are hard to reason with due to their lack of aforementioned empathy, manipulative nature, impulsivity, shallow emotions, and absence of remorse.
I'm sorry you feel that way. Maybe when you get more life experience, you'll realize that most scumbags will only avoid engaging in heinous acts if the deterrent for doing so is appropriate.
Let's say we do decide to torture murderers. Who is going to torture them? Are we going to pay for these people to have therapy (because they're really going to need it)? How would you feel if you found out your next-door neighbour tortures people for a living? What do we do when we find out, years later, that we've been torturing an innocent person?
I think you need a little more life experience so you can realize that things aren't as simple as you seem to think they are.