An antagonist is a
character(s), institute, or concept that is used as a
force of opposition towards the protagonist. Often, in most cases, the antagonist is a bad guy, a villain, however that isn't always the
case. In some cases, the antagonist could very well be a good person, a hero.
Person
1: Those
cops in those movies where they try to stop criminals are good antagonists.
Person
2: But they are good people. How are they antagonists?
Person 1: Antagonists aren't necessarily bad people. They just oppose the protagonists, in which some cases the protagonists is the evil one. Like Death note. Light Yagami
may be the protagonist, but you know he's the evil one and L, the guy trying to stop him, despite him being the good guy, is the antagonist of the
story.