Evil MCs are often quite petty and fixate on those that offend them so it's natural to write them in a way where they don't have to be constantly progressing the main plot to keep the audience entertained. Evil MCs also offer a unique perspective.
They're the one's challenging the other characters so instead of seeing how far the hero's conviction goes you get to see how deeply rooted the manipulation of the Evil MC is. That's at least what makes me enjoy reading and writing about morally flawed protagonists. Most of mine aren't evil but I did say I was going to start writing a dragon story that might be a good opportunity to test one out.