Both Uses

antagonist
in
The Awakening, by Kate Chopin
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The street, the children, the fruit vender, the flowers growing there under her eyes, were all part and parcel of an alien world which had suddenly become antagonistic.†
Chpt 18 *

antagonistic = with opposition or hostility
The children appeared before her like antagonists who had overcome her; who had overpowered and sought to drag her into the soul's slavery for the rest of her days.†
Chpt 39

antagonists = people who offer opposition or are hostile

Definitions:

(1) (antagonist as in: the antagonist in the film)
someone who offers opposition or is hostile
(2) (meaning too rare to warrant focus)
Specialized Definitions of antagonist:
- literature or storytelling: the most important character who opposes the main character or protagonist
- physiology: a muscle that relaxes when another muscle contracts
- chemistry and biology: a substance that interferes with the effect of another substance