@Zencha :
It depend of how you think someone is made (nature vs nurture).
If you think that it's "all in the gene" then yeah, a rapist will always be a rapist from birth to death.
If you think that what makes someone is his experience in life, then you can stop him/her being a rapist if you change his/her past.
In the second case, killing someone that didn't do a crime is morally wrong. (because he isn't a killer yet). But if you know he'll do it in the future, the morally correct thing is to prevent him to do bad by educating him. (and prevent him to become a killer)
Killing can be the best solution if the education process is too long or cost too much. (where you put the threashold depend of how much you value the life and death of someone).
Lastly, I must just correct you that a pedophile isn't a criminal. A child molester is. Unless you agree that thoughtcrimes should exist.
Inbefore anything. (because I know that most people tend to overreact to such subjects). I must say that I didnt' put any personal opinion in the above text apart for what I think is the word's definitions.
My post don't condemn any of the two way of thinking and try to be the most partial possible.
Feel free to correct me if you think I'm wrong.
Have a nice day!