do you personally believe inuyashiki counts as horror genre?
I think it’s a subset of horror, though I don’t know if a term for this sub genre has ever been coined. Attack on Titan is part of the same group, as is Tokyo Ghoul, Parasyte, Gantz and Ajin. I don’t follow anime fangroups, so if a term has stuck for these I’d be interested to know.
All of them take strong horror tropes and stuff them in to seinen or shonen, action narratives (though I think Inuyashiki extends beyond these ages). Parasyte is The Thing/Body Snatchers, Tokyo Ghoul begins with very familiar classic vampire cliches, etc.
I’ve read almost all of Inuyashiki and I see it as similar to the first Terminator, only the old man chooses to do good, and it’s the plucky youngster that doesn’t. Shishigami also fits in perfectly with Japanese serial killer films like Killers (Indonesian co-production I know), Himeanole, Cold Fish and Cure. He’s a villain that’s portrayed how a movie would present a character like that, not an anime.