Why do people want to kill others?!
Question: nope
Answers: nope
Because they are so enamored of their own will and self as to not be interested in considering or accommodating an-others will and the easiest (lazy) way to deal with them is to get entirely rid of them. The solution is to treat others as oneself and develop empathy for their feelings. It is getting outside the prison of self.
Good luck in developing empathy, good mental health, peace and Love!
Because they're crazy.
Because they have a motive to do so, as explained by a police officer or an investigator (power, greed, revenge, escaping blackmail, etc)
Then, from another standpoint, it may how to mere madness, and the difficulty to get life out of someone may vary, depending the mental disease (it is easier for a person with antisocial personality disorder, aka psychopathy, for example)
because they have uncontrolled emotions that have built up and turned to rage!
some people just have so much rage and anger all the time and they keep it bottled up it is bound to lash out eventually