Remathilis
Legend
KoS? Not familiar with that acronym.
But, I do agree with your point here. Horror is easily defined, easy to engage the players and, honestly, tons of fun. The old Stephen King adage about if everything else fails, go for the gross out certainly applies. Like you say, having a clear bad guy makes for a really easy way to hook the players into things.
And, because it's horror, there's nothing wrong with simply dumping scary critters on the party whenever you feel like. Things are a bit slow? Zombie attack! And nobody starts bitching about "Oh, you're just forcing this" because, well, everyone expects to get stuff dumped on them all the time in horror. That's the point.
KoS = Kill on Sight. Most undead, Eldritch monstrosities and demonic entities are pretty safe always Evil antagonist creatures.