Most serial killers have co-morbid diagnosis. The one thing that most hold in common is a diagnosis of anitsocial personality disorder. People with ASD are extremely manipulative, and see others only in the context of what benefit they can be to the them. They are grandiose, and put their desires and their needs first in all circumstances. They value very little but themselves.
They are also able to portray themselves as whatever the situation calls for. That is why serial killers can continue to kill as long as they do. It is a game for them. And they make their victims believe that they are just an average Joe. No one realizes, until it is too late, that there is something to be very fearful of in these people.
The Green River Killer was the most prolific serial killer this country has ever seen. He was able to keep murdering women from the 1980's into the 2000's. He was a married man, and he took many of his victims to his own home that he shared with his wife of many years. Neighbors did not suspect anything, his wife did not suspect anything, no one in his life suspected anything. It took the police decades to actually see him as a suspect and make an arrest. These are not unintelligent people. They don't walk down the street talking to themselves. They are calculating and they plan their murders and choose their victims right down to the last minute detail. They also have a (very logical to their way of thinking) reason for every single murder they commit. It serves a purpose in their lives. It is killing with the motive of providing something for themselves that is missing or has been denied them in the past.
Take a look at Wayne Williams. On the surface, does he appear to be insane? Does he have a history of hospitalizations for psychotic breaks? He was active in a church, helpful to his neighbors, and very meek looking. He killed over 20 young men in Atlanta.
The reason these people are not considered legally insane is because of the definition of legal insanity, which is knowing the difference between right and wrong. The clinical definition is quite different. Did these 2 men know that what they were doing was wrong. Of course they did. That is why it was so hard to catch them. They cover their tracks extremely well due to all the planning. Did they act in the moment of a psychotic break that prevented them from understanding right from wrong? Of course not. They knew murdering innocent people was wrong. But it served the end that they needed to have served, so they did what they had to do to fulfill that need. Does someone who has a compulsive need that is fulfilled by the murder of innocent victims clinically insane? Yes, they are. People who have such a compulsive need such as this are disordered in their thinking and their feeling. We say that people with OCD have a mental disorder,don't we, when they can't leave the house until they have washed their hand 250 times, and locked and unlocked their door exactly 27 times? They do that to fulfill a need. Their rituals serve a specific need. They relieve the anxiety they experience, and have developed as the result of environment and brain differences. Compulsive murderers are enacting a ritual, the same as the person who washes their hands constantly. It is just a more extreme action.