Have you read any H P Lovecraft? He's big on creatures that are so old, so powerful, so far outside our world that they don't really notice humans, except when we go out of our way to attract their attention. And when they appear, they tend to eat a few people, send everyone else present gibbering insane, then go away, completely indifferent to what they've done.
They're not exactly malevolent, just indifferent. They simply treat us as something like we treat rabbits - sometimes cute, sometimes amusing, sometimes delicious, but most of the time, really not worth much thought.
In D&D, a similar example would be the illithid - a creature so mentally advanced that it doesn't believe lesser creatures have real feelings at all. You certainly can't attribute its lack of empathy to stupidity. Rather, it's more like how (non-vegetarian) humans think of farm animals.
Is that "evil"?
I say "yes". And I have no issue with the fact that I'm only saying that because my race is one of those that gets victimised. I firmly believe that, for all the undigested garbage that 3e goes into about "absolute morality", there is always a perspective. I'm a human, I see morality from a human perspective.
It's not that I find the brain-eating thing "creepy", it's that I find it dangerous. Would you hand over your kid to a daycare centre run by a mindflayer?