I'd rather we just kill him, keep his brain intact, and study it no differently than we do CTE patients so we can try and start finding ways to better identify these kinds of people early and stop future ones from happening, because studies are showing their brains are vastly different than normal on the few we've gotten to study.
The truly sad part is, a lot of these violent sexual criminals were also sexually assaulted or molested themselves. It doesn't warrant the behavior on any level, but it shows that it is a cycle that we can stop.