A simple example would be zombies. What makes a zombie more horrifying and frightful abomination than an orc? They will kill you the same way and zombies are probably even a less dangerous threat being slower and really stupid. But one of them is a man while the other is supposed to be a nightmarish violation of nature.
I don't think they kill the same way, unless I'm basing myself on Tolkien's orcs. But orcs are less frigthtening because they'll kill you on the battlefield, they'll have tactics that can be discovered and planned around, they need to eat and drink, they tire, feel fear and will try to avoid wounds.
More importantly, as barbaric as their society may be, it's a resemblance of society, and anything resembling organized society with humanoid looking beings already dents in some manner the fear factor because you start to visualize or try to comprehend them through our own lenses and perhaps can even make meaning out of them that way.
Meanwhile zombies are less intelligent... but completely uncontrollable and illogical. They'll kill you slowly by chewing you to pieces, it doesn't matter if they are slower because they don't need to eat, to drink or even get sick, so stalling in a fortress for months or years is actually worse
for us, who actually need supplies.
But what makes them really frightening is the fact they feel no pain, fear and don't care for wounds received and you'll just keep mindlessly and frenetically coming for you no matter what.
Cut their legs? They'll crawl to you. Made a blockade with fire and spears? They'll keep coming burning and impaled like it's nothing.
Orcs won't do any of that.
Looking like a butchered human corpse does contribute, both for the gore factor, the possibility of someone we know being "corrupted" and forcing us to kill them for something it's not their fault or that they know they're doing, seeing people like us acting like how humanity would look like if we had the sentience level of a virus and perhaps above all, the fact that dieing to them will turn us into the very same thing.
Perhaps this last part is what does it, as it's too alien to rise from the dead, also giving the impression of not only us, but our loved us, the loss of self-control, consciousness, mind, dignity... well their humanity simply stripped away.
Being cut in half by an orc's axe has none of those considerations!