Well, for one, I believe this is the only plane, and generally I don't believe we as humanity could or should look ourselves in the mirror without being disgusted after condoning the execution of sick individuals. I don't really care how evil people think these murderers are, there is no such thing as real evil, there's just what we consider normal behaviour and then there's deviant behavior. The ones who are deviant are by definition not normal. There's something biologically wrong with them so you can't just kill them off for being different. Same how it's not considered justified to execute people who did bad things because of a tumor or because they were mentally retarded. Just because there's no obvious biological pointer for their "evil" doesn't mean it's not something in their brain that caused it.