Okay re-reading my own post that doesn't make a lot of sense even to me
I guess it does have to do with punishment to a certain degree. If you know you would only get 10 years for a crime it is not so bad but if you know that you could be put to death, your risk has just increased, so you might be more willing to kill to cover up your crime, whereas before you might try to rely on pressure to keep the victim quiet.
Honestly I'm not sure a pedophile even thinks in these terms. I know they are very very sick people. Many of the stories I've read and the interviews, they say they know they are a menace to society and they know that if they are free, they will repeat their crime. I'm not sure that someone who is that sick can control themselves. Then again if you are that mentally ill do you deserve to be put to death over something you yourself can't control? Or would life imprisonment be better at a mental institutions. Plus if we have a chance to study them and work with them, maybe we can find a cure? If we just kill them all, we learn nothing about preventing this in the future.