1. It's depends on the specific person. How they grew up plays a big part of it. Some are driven by hate and anger, others by the need for revenge, others for attention, some just want control, some get a sexual pleasure out of it, and others just like the exhilarating feeling of it all. Most serial killers are incapable of feeling empathy, which is why they do not understand the severity of the crime until someone explains it to them. A lot of murderers had some type of abuse as a child, whether it was bullying, being outcasted, emotional abuse, neglect, or phsyical abuse, which sometimes sends them into a world of hate, fear, confusion, and depression. Often times they do not know how to deal with these feelings as they have no one to talk to, and the lack of understanding can cause them to act irrationally or just intensify the emotions. Eventually they begin to have twisted fantasies that become very difficult to control and stop.
2.
- They create art or write about murder and / or torture, or they research it on the internet.
- They are antisocial.
- They torture or kill animals.
- Lack of empathy, have absolutely no concept of how to relate to others.
- Bad grades, shows a strong dislike for school or authoritive figures, also doesn't follow rules.
- Finds interests in others' crimes, natural disasters that cause death, etc.
- Sadistic personality, enjoys pain.
- Frontal lobe deficiency; it's the part of the brain that says, "No, killing your teacher is not a good idea", and holds you back from carrying out the fantasy. Serial killers often cannot distinguish right and wrong.
Then again, there are serial killers like Ted Bundy, who was very charming, smart, and outgoing / social. It really depends on the person.
3. No, they cannot be rehibilitated. They love the thrill and the power they experience during the murder, so they do it again and again. If they have been to prison at any point in their life, chances are they have been de-sensitized to the point where they only feel anger and hate. When they come out of prison they are just like "what the f*ck?!" because the world doesn't stop moving while they are incarcerated. This makes them feel scared and powerless and confused. Also if they had experienced solitary confinement they most likely have developed mental disorders, ex. they are scared of being alone or they forgot how to act around a crowd. With a felony on your record and a long(ish) time in prison, it is hard to obtain a job, which means they have to return to a life of crime. Maybe they had neglectful parents and they just want to elicit a reaction from somebody. Maybe it gets them off. Maybe they love the feeling of control they have over something as important as someone's life. After the first murder, it becomes an uncontrollable urge, a compulsive need. They retain the thoughts for as long as they can, but that only makes it build up stronger and stronger, and eventually they blow and they just need to fulfill the fantasy. Once that happens, everything about murdering people overtakes them until what was left of their innocent personality is gone, and all that is left is the need to kill, the thurst for blood, and the feelings of rage and hate.