Psychology is how murder is explained. People have different reasons for committing murder. It may be pre-meditated or spur of the moment. However it was committed, it happened for a reason and psychology attempts to discover the reason that person behaved this way and what prompted them to do it.
I have the feeling that most murders are not premeditated and that they happen in the spur of the moment, arguing over money, girls, or guys, or any other problem that could be argued in a bar, club, restaurant, or even in a home. I think they can be easily explained in terms of psychology as rage getting out of control.