Very shortly put you could say this is about a young man being unable to deal with rejection.
Tomasz doesn’t turn to complaining to friends of family – he doesn’t have either. Instead he puts all his energy into his new job: influencing (social) media for the highest bidder.
This gets personal when he can get to those who rejected him. The horrors of successful online hate campaigns follow.
Main actor Maciej Musialowski manages to look the sociopath without laying in on too thick, but more about his (original) motivations would have made all this even more scarier and clean cut. Or maybe I’m just too attached to getting questions answered (theme of the month?). Maybe some people are fueled by revenge and chaos and nothing more, turning The Hater (original title is Polish) into a “humans are the monsters”-thriller.