An antagonist is the character in a story whom has evil intent. The antagonist(s) is the opposing character to a protagonist, which are the characters that have good intent. Antagonists normally have reasoning for committing acts of evil. Though it is rare, it is possible for an antagonist to be an evil doer committing acts of which a protagonist might through acts which may seem evil, which throw off most readers and characters of a story.
These are the antagonists in The Vampire Diaries.