Demonic is a word to describe behavior or people who are extremely evil or cruel.
It helps to understand this word if you know a demon is a creature of hell. Demonic has that same kind of hellish, diabolical, fiendish, infernal, satanic, unholy quality. This word should not be used lightly. Stealing a candy bar, though wrong, is not demonic. Truly evil behavior, like being a serial killer, is more likely to be described as demonic. In superhero movies, the supervillains are likely to be demonic, since they are more evil than common storybook villains.