Noisms posted about how humans are more monstrous than any monster you could imagine. Richard replied with an essay about what monsters are for.
I mostly agree with Richard's response, although I can see an argument for why a focus on human villainy is better in some ways. Easy to describe, easy to believe, and if you do toss in a rare non-human menace, it will stand out all the more.
But I disagree with Noism's premise that human monsters are worse than imagined monsters. Unless you are doing a crappy job of characterization, a human monster is at least theoretically able to be reasoned with, or threatened, or appeased, or maybe just able to be swayed emotionally in one way or another. And if your "monsters" are just humans disguised as goblins or the like, human monsters will be a better choice.
But what makes a "real" monster monstrous is not the horrible physical things it does, like murder and torture, but the fact that it is implacable and uncanny. We've trivialized the undead a bit, but there's still a horrific core to the concept that no historical inhuman behavior has ever come close to. Sure, torture, cannibalism, and murder are frightening, but think about how you'd react if you really saw an actual skleton walking towards you. It would be far more frightening, even if it never threatened you, because it violates everything you think you know about the universe. And if it does threaten you, somehow you know that threatening it back with bodily harm isn't going to help. How do you kill the dead, assuming there's no guarantee that any legendary methods even work?
The same applies to any other kind of "real" monster, in the sense of something that violates your understanding. We're jaded, because we've read every Monster Manual and memorized the stats, strengths and weaknesses of various monsters, but all you really need to do to top any purely human horror is remove any certainty.