It fulfills their fantasies of domination, manipulation, and control. Murdering someone is, after all, the ultimate in domination, manipulation, and control. It's also the ultimate in selfishness. Ultimate selfishness and pure evil are joined at the hip. Douglas, says that serial murderers are generally losers.
None are able to maintain a mature, consensual relationship with another adult.
Since they are unable to dominate, manipulate, and control enough to satisfy their desires in the ordinary world, murder becomes their outlet. Some of them, it seems, have become icons of the Left. You've probably noticed how uncompromising the left is in wanting to spend more of your money. Spending your money provides them with the power to retain power. No way are they willing to give that up, given that it would mean decreasing their control.