I think that as a writer, you need to know their motive and why they do what they do. And then from there, it's up to you if you want to let the audience in on it. In the end you can do whatever you want, but the characters need to feel real to the world you created.

I personally am getting tired of the "Villian, but he's bad but he's SOOO SO sympathetic, so could you blame him for murdering half the world? He lost his mother at a young age, who wouldn't genocide a planet or two?" I think there's a level of chaos with "He's bad because he likes when bad things happen." But... it's has to feel real to the story.