Well, if we knew the answer to that, there'd be a lot more people alive and well today.
But nontheless, criminologists and psychologists are constantly fascinated with the question of what drives someone to kill, and kill again.
The vast majority of murders are one-offs, owing to opportunity, 'a crime of passion', or for a particular premeditated purpose. So what sets apart those, such as Dr Harold Shipman (pictured above), who kill repeatedly?
This new infographic created by Best Counseling Degrees takes a look at patterns, traits and statistics borne out by a study of historical serial killers.
Take a look below and see if you can put the pieces of the puzzle together.