I don't think it's wrong to kill someone who's raped and murdered a 6 year-old (for example). It doesn't make it right, but I don't see it as being wrong either.
Although in most cases, I'd say keep them alive to live with what they've done, as punishment, as I don't think death is severe enough. Death is an easy way out for someone like that. But they don't have it hard enough in prison. I don't mean in general, I mean the ones who REALLY deserve to suffer. tbh, there are some kinds of people who, if it was up to me, would just be put in a cage and tortured. But (luckily for some), it's not up to me.