The lecture "What makes a bully" by psychologist Dr. Gordon Neufeld describes the psychological reasons for people finding joy in the misery of others. It's so good I put it on my youtube channel. Very long, but well worth watching.

The tl;dw is that a need to feel dominant, compounded with a lack of empathy, makes people feel good when they can make others miserable.

The lecture describes how these issues arise, and also how they can be remedied. His work is geared towards childhood and adolescence, but the same things happen with adults too.