See, all the best villains have back story, right? And there are few villains, particularly good ones, who don't believe that what they are doing is right. The notable exception to this is The Joker. But really, every character is simply trying to get what they want the best way they know how.
Because of this, I pity my villains and want them to get what they want, and because villains are layered and complex I usually end up rooting for them way more than the boring, unrealistically perfect hero types.
This is why Severus Snape is my ideal character. He's definitely not a hero in the traditional sense, but he's not the villain either which puts him in the anti-hero space, which makes his efforts at goodness way more heroic, in my mind, than it would be otherwise. He had to work for it, consciously choose good over bad. Although to be fair, Rowling does a pretty darn good job of making all of her characters complex, including Harry.
Basically what I'm trying to say here, is that when I try to write villains they always end up more Snape than Voldemort. Which I suppose isn't a bad thing, unless I really want a Voldemort. But really, we all just write what we write, right?