As a general rule, evil—pure, emotionless, motivation-free evil—doesn’t work for your antagonist. Or for your protagonist, for that matter. You need sympathy, you need realism…bullshit.
Okay, not complete bullshit, but still: so many times, pure evil does work. Yes, having a well-rounded villain is all kinds of good and stuff, and can work out beautifully. But why does it have to be a necessity.
I turn your attention to The Nothing. It simple is…or, rather, isn’t. Yes, it comes from the lack of human care for the imaginative, but The Nothing itself is without emotion or reason. It’s like the Black Death, or a honey badger: it doesn’t care.
Up until we find out about the prophecy, Lord Voldemort doesn’t have any motivation for continually attacking Harry other than the fact that he’s evil. He has clear reasons for doing things other than attacking Harry, but not for the main conflict of the series. Did that make him any less effective? No.
This isn’t me saying that pure evil is the only way, but it is me saying that, from time to time, maybe it’s okay to have something or someone that’s just evil to be evil. Like cheesecake—eat it from time to time, but not every day.
Where do you fall? Are you okay with pure evil, or do you require that motivation?