fiend

An enemy or an evil person is called a fiend.

When we talk about fiends, we often think of dark and scary things. In movies, books, and games, fiends can be supernatural creatures like demons or ghosts that are out to harm people. If someone in your neighborhood is causing trouble all the time, they might be seen as a local fiend. The word can also imply malevolence, so if you're talking about a person who has committed some terrible crime, you could call them a fiend.