What about a terrible evil that is just...a variety of people? That's what I have. The Raven-Shadows are as diverse as the Safiqi priesthood whom they broke away from, long ago. The Shadow Druids (no relation, indeed no love is lost between the two groups) are carefully recruited from the population of regular Kahina, so they have no reason to be any less physiologically diverse. The Cult of the Burning Eye is older than formal human civilization, so it of necessity is not human or non-human but both and neither. And the gang run by the black dragon trying to take over the city? It couldn't possibly care less what race you are, it just wants footsoldiers in an underground war of control over money and infrastructure.
Humans can be evil. Non-humans can be good. Only those who have some clear reason to be Always Evil are so. Like the immortal supernatural beings who fought an infinitely long war of ideology for the right to force mortals to obey The Plan. Or the disturbing abominations from beyond the stars that choose to eat the brains of sapient beings in order to feed themselves. Or the hollow shades of once-living men and women, that consume the blood and life of the living in order to delay Death's embrace for ever-shrinking periods. These are Always Evil because, if they chose differently, they couldn't be the things they are. And they definitely aren't player-character material (being "devils," "mind flayers," and "vampires" specifically, though no vampires have yet appeared in the game.)
Neither the players nor I can be lazy. We can't presume that a dragonborn is a good guy or an orc is a bad guy. It's only things that are blatantly one thing or another--something so far beyond the pale that there's no going back--that you can respond with "kill it!" and not be an awful person. And even the "bad guys" often have a great deal of nuance! The Raven-Shadows are a literal murder cult, who find transcendence in the moment of a person unexpectedly facing Death. Yet they have also defended the mortal world from HORRIBLE nightmarish things that would have loved to infiltrate and destroy mortal-kind. The black dragon gang's low-level members are often just poor folk struggling to make ends meet, who get an amazingly generous offer at just the right time, and don't realize the error of their ways until far too late. The Shadow Druids--those who haven't been absorbed into the fungal hive-mind, anyway--are disgruntled, disaffected folk who pine for an age that was brutal but in which the Kahina were the succor and guide of mortal-kind. Only the Burning Eye cultists aren't nuanced, and that's just because I chose to make them (essentially) Cthulhu worshippers who throw themselves into blood frenzies; the rank-and-file aren't so much evil as truly insane, with only the thinnest veneer of sanity keeping them from being discovered.
Even "always evil" can be nuanced. My devils aren't like ordinary ones. Frex, D&D devils are lying, cheating, swindling, rules-lawyer idiots who embrace their reputation for conniving lies. Mine know that a rep of cheating/lying costs you business, so they're hella scrupulous...with business. Further, each devil WANTS every contract completed. Contract failure is failure, with a lame consolation prize (a mortal soul). And devils don't give blatantly offensive contracts. They're almost always tailored to the signer, so they'll want to complete it. (E.g., assassination contract to kill wicked devil-worshippers that have killed children.) Outright repulsive contracts are colossally stupid, and thus always avoided. This is smart, effective evil, not Dick Dastardly losing the race because he can't bear to win fairly.