I see your point, but the biggest thing that hit me here was that this guy was masturbating (at least I'm pretty sure that it's implied, and if not I'm reading into things way too much) in a court room watching the trial of a serial child-killer
this isn't just some weird guy with strange habits - this is someone with a serious, serious problem
maybe he is just a talker or maybe he really will go out there and fulfill his twisted little fantasies by killing people
I don't like that the main character just broke into this guy's house and killed him. I'm not saying that everyone with any sort of a weird kink should be overly scrutinized (cause that would be waaaay too many people, myself included). I was just remarking on how difficult of a situation this is. Imagine if you were in court as a witness to a murder, and you saw some guy enjoying himself in the back. Obviously you're not going to put on a costume and go kill him, but what can you do? You can't tell the cops - he's not doing anything illegal. You can't follow him and monitor his every move (well, you could, but that wouldn't be too healthy either). And yes, you don't know for sure that he might go out and do something terrible.
But if he's jerking off to descriptions of a crime scene, I would say that there's a pretty good chance that he will. So how would you feel walking out of that courtroom? Could you honestly tell yourself that he's probably just a nice guy with an unusual trait? Would this not bother you for a good while? Would you be a little worried when you read the local news every morning?
I'm not suggesting a solution (like I said earlier, killing him without him having done anything seems very wrong), and I sure as hell don't know a solution. I was just trying to voice my unease of this kind of a situation, and I hope that I never run into it.