Should a murderer be killed

If someone is genuinely evil and has committed truly evil deeds, it is then okay to kill, the problem with this is that there are few who truly believe they are evil and have their own reasons that justify what they have done. Hitler believed the Jews to be evil, even though just about all of us disagree with this including me, he never believed what he was doing was wrong. But Hitler is evil in my eyes and thus i would kill him given the chance.