I always thought they were horror. In Resident Evil 5, there was heads blowing up with some dogs with their heads being torn apart, maniacs with chainsaws, willing to rip anybody apart, some gigantic monster that used to be a human, and much more.
And Resident Evil 4 had almost the same thing, except for experimented on humans that are the most creepy thing ever, and with some having sharp, black objects stuck in their face. Oh yeah, and a man with his chest ripped apart, and a midget that gets turned into a gigantic monster, that shows his body, with odd things on it, the leader of the entire place turning into a disgusting monster.
Really, it doesn't have to have zombies just to be creepy.