You're right. I have checked around on some of the databases, and it looks like they are, for the most part, offenders who are more likely to re-offend.
So, I'll change horses in mid-stream and join the minority who think the "burn them all" vigilante crowd might have solutions in mind that would be very bad for justice in general. Specifically, I'd suggest that if one of their own children committed one of these acts, for example, they might change their mind and decide that the 5 or 20-year sentence was punishment enough. Punish people for the crimes they commit, not those they might commit.
If one of my kids did that, I'd be asking the judge to impose the death sentence myself. Its the least I could do to try to make it up to the victim(s) and their families for spawning the horror that my child had become.