It's fascinating how Hatred doesn't do anything new in the violence department, but because of some details it pushes the right buttons to piss off modern moral guardians who would be ok with this game if it had a different tone and style (see Hotline Miami).
The main character doesn't seem to have any motive for slaughter except for misanthropy, but he looks like someone that could be posting on /pol/. I can imagine sjws fearing that Hatred could be enjoyed by angsty white edgelords who would LARP that they're punishing the degenerate, liberal, politically correct etc society. Or maybe it's just the reminiscence of the Columbine guys, trenchcoat, edgyness, metal (well there's no metal or rock in the game but the devs look like metal fans). The realism and lack of jokes, funny memes, popculture references and other stupid quirky, nerdy crap probably also contributes to the butthurt.