There are a large number of studies that show that places with the death penalty encourage crimes to be more violent. For example if I'm going to rape a child - at this stage being my first offence I will probably get in reality about 8 years in jail in Australia (and I think 8 years is a really long time - it's longer than I've been playing footbag for a start). Anyway if that was changed to the death penalty than personally I'd rather the lawyers prove that I killed the child rather than trusting the child not to tell anybody. Logically I would say it is in the best interests of pedophiles to kill their victims if either way they're going to get the death penalty and the studies show that this is what happens as well. I'm sure if you search you can find results that back up what I'm saying.
It comes down to that question that was posted somewhere else of intent vs impact. The intent of introducing the death penalty for pedophiles might be to make sure they don't do it again. The impact is that children are killed who otherwise wouldn't be.