Why do people commit murder? (Seong Kyung Kim)

People kill for numerous amounts of reasons. One motive may be revenge. Usually, when someone kills out of vengeance, they do not think clearly, resulting in the instant decision to kill based on anger. Though most vengeful killings are not thought through or premeditated, some killings are. Certain vengeful killers filled with jealousy, anger, hatred, or a hunger for revenge may plan the definitive killing of a person or group of people using an intricate plan thought out for days, months, or even years. Although revenge may be one of the main reasons people kill, it is not the only one.

Mostly murders have unfortune background. A common belief is that people with a metal illness are dangerous because they can suddenly become aggressive. In reality, they are no more dangerous than anyone else. It is true that, in some instances, the symptoms of a metal illness can lead to aggressive behaviour, but this is rare.
(book)-Where there is no psychiatrist: a mental health care manual By Vikram Patel (Pg.53)

http://everydaypsychology.com/2008/07/why-do-people-kill-typology-of-violent.html
-At various times, I have been asked to lecture on the topic: why do people kill? Why do they commit murder? How do we explain acts of violence. The following is not an exhaustive explanation. It's a handout I use when I speak about different types of killers and the motivations for different crimes. But it describes many, if not most types of homicide and most acts of violence.It describes most of the killers and most of the violent individuals.

http://www.senia.com/2006/09/12/why-do-people-kill-people/
-A story based on a true story, 'why people kill people?'.

http://www.angelfire.com/zine2/narcissism/antisocial_sociopath_psychopath.html
-People who cannot contain their urges to harm (or kill) people repeatedly for no apparent reason are assumed to suffer from some mental illness. However, they may be more cruel than crazy, they may be choosing not to control their urges, they know right from wrong, they know exactly what they're doing, and they are definitely NOT insane, at least according to the consensus of most scholars (Samenow 2004). In such cases, they usually fall into one of three types that are typically considered aggravating circumstances in addition to their legal guilt -- antisocial personality disorder (APD), sociopath, or psychopath.

http://cityroom.blogs.nytimes.com/2008/02/19/murder-case-focuses-new-attention-on-mental-illness-treatment/
-The arrest of David M. Tarloff, a man with a long history of mental illness, in the fatal stabbing of a psychologist in Manhattan has revived discussion on the thorny issue of how people with severe mental illnesses can be helped, even when they resist that assistance.