The murderer in your dream is most likely a part of you that forces changes or endings in your life. The positive aspect of it is that you are putting an end to that which is not useful to you. The negative aspect could be that you are sabotaging a relationship, opportunity, etc.
If there is a serial killer in your dream then this points to the recurring behavior of quitting or ending things before they are finished.
If this description does not fit you then this may be about someone around you that is currently affecting you.