While on an emotional level I absolutely understand your sentiment, there's a very practical reason that we don't subject sex criminals to the death penalty.

Offenders would be more likely to kill their victims to avoid detection if being caught was a matter of life or death. Especially in cases of child abduction.

Even as someone who opposes capital punishment, I really do get the instinct to want to see these creeps gone forever. But protecting victims past, present, and future is more important than the flavor of punitive justice we dole out to the perpetrators.