Definitely murderer, as it's more real. The more real something is, the more scary it is. Monsters aren't real. That's why some war films are also some of the best horror films.
Ya know, I feel the opposite.
Even though I don't believe in the supernatural at all, because I am a 200lb. man, I don't feel nearly as threatened by some killer. If you put some dude in my apartment, I have a chance, who knows, maybe a good chance that I come out on top. But you put a
witch in my apartment (even though I don't believe in witches)... god knows what's gonna happen to me. I mean, I got something against some guy with a knife or even an axe or something, but against a
witch? I got nothin'.
I think this is actually why slasher movies are so famous for the characters making impossibly stupid decisions. Because if the villain is human, then they should actually have a pretty decent chance of survival. In fact, if they're a group and the villain is just one, then they actually should survive with minimal losses, if we're going with "real" as you said in your post. It is necessary that they be stupid AF for there to actually be danger. This is why you never read stories of mass killings with an axe.
I used to have, way back in the RT days, Wooley's Rules For Surviving A Slasher. And they were pretty obvious. I don't remember them all but some again obvious highlights were:
Run. And not like an idiot.
Run in a straight line as fast and far as you can without stopping.
Don't trip over things.
If you have found dead bodies already, don't investigate strange noises alone and unarmed.
Never choose the flashlight over the shotgun.
Don't drop the shotgun/shovel/whatever just because the killer appears to be down.
If the killer is down finish him.
There were many more, but all of these will save you from any human so you can't do any of them in a horror movie or you will have no movie.
Furthermore, all the other tropes that make the genre a punchline most of the time are actually necessary for the killer to actually succeed in these films. Like the phone never working. The car doesn't start. The killer is super-human when necessary for the story, at times being preternaturally strong or preternaturally resistant to injury and pain or has the apparent ability to teleport or the very unique ability to slashers and serial killers of being able to hide several bodies convincingly in the space of minutes. Or the movie will simply change its own rules to allow the killer to be successful or more threatening when this would not be possible by the rules that govern reality.
And I think it is because the rules of reality are out the window in a supernatural horror film that they are more frightening.
It would suck to be the kids who got caught in
The Texas Chainsaw Massacre but, to me, that is a rare film. But if movies are fantasy so that Leatherface and Cannibal Family can be a thing, I like my chances escaping them a lot more than I do escaping a ghost or a witch or, god forbid,
the ghost of a witch... or some other spooky thing.