Evil, savage humanoids

My cosmology removes Good and Evil in favor of Chaos, Cosmos, Discord and Order. Of those, cosmos is the only allegiance which is not inimical to life as we know it. The evil, savage humanoids in the MM are re-imagined as agents of Chaos.

My setting uses a single pantheon for the entire world rather than race specific pantheons. Humans and demihumans worship the gods (Cosmos), humanoids worship the demons (Chaos). What makes the monstrous humanoids psychologically distinct from humans and demihumans is that they are predisposed by nature/nurture towards serving Chaos. This means that their driving goals are to tear down civilization, pollute nature, and generally cause mayhem and suffering. Their reason for doing this is fairly rational: in hell your status is determined by how evil you are (by comparison, humans are nice to other humans out of a combination of empathy and desire to go to heaven). Since committing atrocities against their own tribe is counterproductive, humanoids generally commit atrocities against outsiders.

What exactly does it mean to be inclined by nature and nurture to serve Chaos? It does not mean they lack free will or have it limited, despite what the MM may say. One of the conceits of my setting is that corruption, the currency of hell, may only be accumulated if the actor is free-willed and knowingly causes harm. (BTW, I am using the corruption mechanic from Mongoose's Book of Hell.) Humans and demihumans all have a conscience and an id. The conscience is a voice in their head that tells them to do the right thing, while the id tells them to pursue self-gratification regardless of the consequences. Humanoids are born with an inverted conscience which urges them to serve Chaos, cause unnecessary suffering, and ultimately destroy reality. This cheats the prerequisite for free will and does not absolve them of responsibility for their actions. Where a human born in a deeply misogynistic, racist culture will turn out completely different if raised in an egalitarian culture, a humanoid raised in human society will constantly struggle with their inverted conscience.

What makes humanoids biologically distinct from humans and demihumans is that they don't really have any kind of childhood. They are generally "born" as adults (or miniature adults) with sufficient knowledge to survive. This saves energy which would otherwise be spend on raising and educating children. In many cases they do not actively reproduce, but generate spontaneously in response to certain stimuli.