A question that has occurred to me many times. It is a very difficult question to answer as every murder is unique. However, inevitably, there are many theories out there one of the main ones being that a killer is driven by his past:

If you are anything like me then this may at first seem like an excuse, however, it is well known that until the age of about 18-20 you are the dough that can still be molded into many shape; meaning you are still flexible with your mind and experiences. Along the years you gain, from what you witness around you, a personality. It is often found that killers and psychopaths have a troubled past; divorces, grieving, abuse, addictions. All of these experiences embed hate and rage into the victim and can (depending on the strenth of the victims personality) push to do actions that match up to his emotions (like murder, robbery, rape) as a way of trying to let them out.