Talysman, over at Nine and Thirty Kingdoms, has highlighted a post over at Throne of Salt which makes the claim that D&D doesn't know what monsters are. Their conclusion is that monsters should be a symptom that somewhere, somehow the world has gone seriously wrong and that the way D&D does monsters fails in this regard.
Now, I cannot speak about later editions of the game (though I can believe this of 5e, which seems to have forgotten how to teach players how to play the game), but I can speak for 1e. The MM1 was one of my very first purchases when I got into the hobby, and I spent many hours as a kid being inspired by what was depicted therein. I can, without hesitation, say that the MM1 explicitly and implicitly depicts monsters as symptoms of something gone horribly wrong.
Take the entry on the Bulette:
The bulette (or landshark) was thought to be extinct until recently when this horror reappeared. It was a result of a mad wizard's experimental cross breeding of a snapping turtle and armadillo with infusions of demons' ichor.
Some idiot magic-user went and pulled a Frankenstein, but we took care of it. What? The monster is still out there? Who has been mucking with magics that shouldn't be messed around with?
Or, how about the Ghoul:
Ghouls are “undead” once human creatures which feed on human and other corpses. Although their change from human to ghoul has deranged and destroyed their minds, ghouls have a terrible cunning which enables them to hunt their prey most effectively.
Couple this with the entry on Ghasts, which boast an Intelligence of 11-12, you have the makings of a cult that seeks to cheat death through cannibalism. While functionally undead, these creatures are actually human.
Of course, there is the Owlbear:
The horrible owlbear is probably the result of genetic experimentation by some insane wizard.
Those pesky wizards, trying to play God in their towers, churning all kinds of vile things into the world.
My favorite examples, though, are statements like this one, under Hobgoblin:
If elves are nearby, hobgoblins will attack them in preference to any other troops because of the great hatred they bear.
Similar statements can be found under kobolds, blinkdogs and displacer beasts. These statements invite us to imagine why such hatred exists in the first place — something is terribly wrong and these monsters are personifications of it.
So, for those of us who want to have archetypal evil in our FRPG worlds with monsters who personify sin, D&D has done us right since at least 1979.