It was that simple for my grandmother. She was tough as nails and had seen a lot. I wouldn't want to be on the opposite side of a battle with her, but tough didn't mean that she would endorse any penalty for criminals. She was against the death penalty, because she followed the Sixth (the Fifth, in some views) Commandment, Thou Shalt Not Kill.
Actually, the Commandment isn't really thou shalt not "kill." It's thou shalt not "murder." Even so, Nana believed that the deliberate taking of a human life, outside of self-defense (and the collective defense fits, I think), was wrong. So she was against the death penalty.
My view isn't quite so direct. There are some unspeakably brutal criminals in this world. For a select few, I can't say I'm against the hastening of their demise, at least in principle. My problem is more practical: human beings make mistakes. Having represented the police in court, I understand how many detectives work. They sincerely believe they have the right suspect, so they build a case to get a conviction. They don't mean to railroad anyone, but they're going to do what they have to to get the bad guy. That's part of the problem, but there also are bad witnesses, mistakes in court, unequal levels of legal representation, etc.
This may be another case of the wrong man being sentenced to death. There have been others before. As each story surfaced, various people tried to assure all of us that there was no evidence of innocent people having been put to death in the past. Really? I think we, as a society, just don't want to look too hard. Sadly, I'm sure it has happened.
As bad as it would be to send an innocent person to prison, at least there is the opportunity to make amends to some degree if an error is found. Death is forever. I have no doubt that the cops, the witnesses, the judge and the jury believed they had the right person, but people make mistakes! That's all the reason I need for saying we shouldn't be killing people, no matter what we're convinced they did!