This person deserves the death penalty. That is very clear. It is quite simply perverse to sit and think up more torturous ways to do it.
As far as how sex offenders, in general, should be treated... well, you come up with a system that doesn't have to rely solely on the word of an individual or define groping a woman as a sex offense and I'll listen. Right now, the system is so far away from any modecome of common sense that I can't see the death penalty as punishment for anything but murder.
But this thread wasn't really about that. It was about how cruel the execution of this individual should be. Cruelty is not a remedy for anything.