The reason serial killers plead insanity is because they hope they'll have a lighter sentence, or that their sentence will be completely revoked. Of course, neither of which, is likely to happen. It could also be because they're simply trying to justify their actions. Of course any individual, who is a serial killer is demented and morally corrupt, to say the least. Clearly, serial killers have mental issues. Although, the legal definition of insanity basically means that one cannot understand the consequences of their own actions. Personally, I really doubt that anyone is dumb enough to be unable to fully understand the consequences of their actions; unless, they are severely mentally-retarded, or they're high on drugs. Otherwise, most human-beings are completely aware of their actions and the consequences of those actions.

A lot of serial killers claim diminished capacity (or something similar) rather than insanity because diminished capacity and the like are less risky in terms of a defense. To prove, in a court of law, that somebody is insane is easier said than done. From http://en.wikipedia.org/wiki/Insanity_plea: The public tends to believe that the insanity defense is used more often than it actually is, possibly because insanity defense cases tend to be of a high-profile nature. The insanity plea is used in the U.S Criminal Justice System in less than 1% of all criminal cases, and only one fourth of those defendants are found "not guilty by reason of insanity". I agree that pleading insanity, if one is a serial killer, seems like the obvious thing to do, but as insane as they appear. . . .the question is usually do they know and understand right from wrong? Apparently no matter how demented a serial killer is, he/she isn't insane unless they have zero grasp on reality and do not understand right from wrong. Naturally, this rule of thumb does not apply in every single jurisdiction in the universe, I am not an attorney, and this is a much more complex issue than one would think. Hope this helps. Check out: http://en.wikipedia.org/wiki/Insanity_plea (lenghty) or http://www.crimelibrary.com/criminal_mind/psychology/insanity/1.html (shorter)

Insanity, when used in court, is a legal term, not a medical diagnosis. Of course serial killers are broken in a way that makes them capable of things a normal person could not do. But legally, if they are aware their actions were wrong and/or took measures to cover up their actions, then legally speaking they are sane and competent to stand trial. As to why they plead, it's usually an attempt at a lighter sentence. However, I read once that only in about 1% of all defendents successfully have a pleas of insanity entered, and of those, about 1% are actually found insane.

Excellent question lady fuschia, and I agree entirely that a serial killer must be insane to carry out such heinous crimes. To me they are obviously psychopathic, which by definition means that they must be afflicted with a personality disorder characterized by a tendency to commit anti - social, violent acts. The actual murders are most likely to have been committed whilst under a psychosis, which means that their sense of reality was highly distorted at the time. These two points are recognized as symptoms of a various mental disorders, so why not accept that a serial killer is mentaly ill? After all mass murder is hardly a sane hobby to partake in is it? I accept that insanity becomes legal as oppose to medical jargon in a court of law, but I personally don't think doing bird in an asylum would be any peachier than prison. Having to share a cell / ward with Ian Brady or the Yorkshire Ripper is unlikely to be a bed of roses is it?

No. Serial killers are perfectly sane. They wouldn't do something so premeditated if they weren't. They would be a spree killer instead. The problem with them isn't insanity. It's being sociopathic.

No. some people actually just enjoy killing people. that does not make you crazy

No, that's not a given...Some serial killers have done their acts of violence out of sheer hatred and many other reasons that they can only bring light too of course your right, there are many that are insane or psychopathic so I guess it's a little of everything that has been mentioned here...

Serial killers are rarely insane. They are calculating, narsistic, amoral individuals, they know exactly what they are doing.

On the contrary with other answers, I do think you have to be seriously mentally ill to commit murder and totally insane if serial :( Pardon me, but from where I stand, just physically hitting someone is total unruly behavior!

Yeah, I would assume that for someone to be a serial killer one must be insane, but strangely courts make them prove their insanity or sanity. If you are insane you don't go to prison and if you are considered sane, which is in a sense, being aware of your insanity (which ignores the concept that the insane are insane because they don't know they are insane). But to plead insanity makes the difference between going to a prison or a mental institution so I can see why someone facing prison would choose that option.