You're completely missing the point and focusing on an individual. They're stating the moral purpose of a SOCIETY. As in the society shouldn't kill and it certainly shouldn't be ONE individuals decision for that murder to take place. YOU might have a redline/breaking point, but society wouldn't. It's why ONE person isn't the deciding factor on death, it's a societies moral choice to do that and uphold that.
You guys can pretend that EVERYONE has a breaking point, that doesn't mean you can't have a society that doesn't have the death penalty. I feel like these responses are just people trying to incite continued violence or justifying their own extremely vitriol need to kill those they deem less.
People can downvote you but aren't even thinking it out. Hitler right now is still a projected person for the far-right nazi movement and is brought up constantly. What if he had been imprisoned and actually got mental health care that doesn't really exist in most prison populations currently (globally that is). If you had a senior Hitler, with life imprisonment, painting fields of flowers with jewish and little blonde/blond kids running around, it would be a totally different outcome in this day and age.
To be possible though the prison system would need completely reworked. In our current system I don't think it would have the same outcome (since our system has a different purpose than rehabilitate currently). I also think people shouldn't be able to communicate as effectively with the outside world without extra censorship (that whole no harm to society thing, can still happen if they're voicing action or calls to violence, happens still currently.).