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contrary adj
perversely inclined to disagree or to do the opposite of what is expected or desired: she is sulky and contrary where her work is concerned.

adversarial - pertaining to or characterized by antagonism and conflict

Oppositional (adj) came to mind. Also the phrase arguing for the sake of arguing.

And some of those people might call themselves a devil's advocate. As this link shows, however, a devil's advocate takes a position he or she doesn't necessarily agree with for the sake of argument (not for the sake of ruffling feathers). The purpose of playing devil's advocate is to test the validity of the other side's arguments.