There are many dimensions that apply to the thought process of a person that kills. Myriads of researchers throughout the ages contribute it to eight basic dimenisions that "influence people to kill.
1. Social environment
2. Family background
3. Peer group association/personal relationships
4. Contact with defining agencies, law enforcement, psychologists/psychiatrist, other agencies
5. Offensive behaviors
8. Recall of events
Now, these dimensions are not all inclusive and there could be more added in the future. If you take a close look at these dimensions, they tend to overlap one with each other. Take for instance Jeff Dahmer, he had a horrible childhood, resinted his mother, had self esteem issues, recalled past events, had a poor attitude, and so on. So, to answer your question, there is no one single reason that people kill. It's an intertwined and tangled web of psychopathic, biological, and sociological deficiencies that occur in the mind of the killer.