Mean All Along

That depends on the stories, because the heroes thought the antagonists or villains is one of the good guys turns out to have always been mean. Not all mean characters are evil because they usually neutral characters.

For example, the hero meets a very nice person who offers to help the hero For whatever reason. If the movie is constantly trying to deceive the audience like this, it can be harder for the audience to get to know the antagonist.

This is when the character was always bad and faking good, not a fall to the dark side.

If the movie is constantly trying to deceive the audience like this, it can be harder for the audience to get to know the antagonist.

See "Hidden Evil" for subtropes. Not to be confused with "Face–Heel Turn", which is a legitimate good guy becoming mean.

Antagonists Gallery
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Villains Gallery
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