Wow. Kagus you're advocating revenge through the legal system, and it's a separate debate about what that's for. Personally I don't think revenge has any part in justice. But at the same time you seem to be suggesting that the fact of a crime is irrelevant beside the mental state of the person that may or may not have done it. Basically, that a genetic predisposition to mental illness, or real mental illness, constitutes guilt.
There is no 'murderer disorder'. There aren't really any fixed disorders at all, except that psychologists love to classify things in neat groups. If you can point to a gene, or several genes, and show that every person that has those genes is a cold blooded killer, or a drunk driver, or jewish (sorry, had to throw that in there because it's about as ridiculous), I will eat my foot. If you know that this isn't the case, and I greatly hope you do, what gives you the right to say that, for instance, a person with severe schizophrenia coupled with pyromania is guilty before ever having committed arson?
If mental illness can't account for crime (no really, it can't. there are always other factors, and it is always more complicated), then the only other thing left influencing behavior is the interplay between a person's environment and that person's mind/body. This is what I was talking about. I won't for a moment deny that these things are a problem, and that there should be some means of controlling it, but the moment you exclude these people from the grouping of 'human', you let go of the key variable.
The fact is, no serial killer emerged from a vacuum. Rapists were little babies at one point, and their feet were ticklish. They influenced people around them the same way they were influenced themselves. The term 'monster' implies some unknown, but for the most part, we can know these people, and it is irresponsible to refuse this opportunity in favor of culling them when they emerge. If they were robots programmed to destroy, doing that would end the problem forever, but the real issue is that the society you live in is where they came about.
Do you really think your kind of vengeful thinking is any different from what goes through a killer's mind when he decides to kill someone that 'wronged' him? The only real difference is that it's backed up by a large number of others. The argument that crime must be treated differently from other human behavior is really kind of missing the point - you can put anything into that slot, and vengeance will go on working the same way. Devoting resources to vengeance is time away from looking for what grew these people, and educating people about how to prevent the same conditions from happening again.
I wish I could talk more, but classes start soon. I hope you take what I say into consideration. I don't have much hope of that, but I kind of feel like I should try anyway.