Use the adjective diabolic to describe someone who acts in a terribly cruel way. A diabolic boy might pick up a cat by its tail and swing it around.
When people are diabolic, they're evil. The horrible dictator who has thousands of civilians killed commits a diabolic act, and people who abuse children are also diabolic. The bad guy in a fairy tale is always diabolic. You can use diabolic interchangeably with the slightly more common diabolical. Both words have a Greek root, diabolikos, which means "devilish" and comes from diabolus, or "devil."