For the very worst cases - serial killers, paedophiles and so forth - I don't think they should be able to return until a panel of criminal psychiatrists are convinced that they won't reoffend. But in all these cases, it is very easy to undo the damage caused by the sentencing if it turns out they are innocent.
From an entirely reptilian, questionable ethics stand point I think that the death penalty for those kind of cases is one of the most scandalously wasteful answers possible. These people are incredibly valuable, to neuroscientists, to psychologists, even to insight for criminal investigation (Ted bundy was an excellent resource to be tapped).
If rehabilitation or preventing further danger to society is what is desired then destroying these commodities in the name of revenge masquerading as justice is counter productive.