I don’t really know whether you’re looking for demon MC’s, or antagonistic MC’s
So here’s a list with a few MC’s who aren’t human, and are antagonistc
B: the beginning
Blassreiter
Devil’s line
And here’s a list with antagonistic MC’s who are human (but have superpowers)
Code geass
Toaru kagaku no accelerator
Arifureta
There’s also an anime called Juini Taisen, but that one doesn’t really have a hero, and it’s more about antagonists/bad people fighting each other in a death match like setting