If an animal kills or severely harms a person we tend to put them down. We also accept that they didn't decide to kill or harm a person, they just did, as they are an animal and can't make conscious sentient decisions.
But if a human kills someone, or severely harms them (including the emotional trauma associated to crimes such as rape and paedophilia), and we know they made a decision as a human being, then suddenly it's a difficult decision to put them down.
Humans aren't sacred. People as a whole have no qualms about killing other humans. To kill someone who has killed others (or made equally atrocious crimes) doesn't make you as bad as the person you are killing.