In my novel, one of the main characters is bad. Not in a nuanced, complex, a-few-nice-traits-that-are-difficult-to-reconcile-with-the-rest-of-his-life kind of way. He's just a monster, who happens to look like the rest of us (think Matt Damon in The Talented Mr. Ripley). No matter how I try I can't find any softer qualities that might make him multi-faceted or interesting. He just goes after what he wants and won't stop for anything or anyone.
He's only part of the story, but I'm wrestling with how to write him so that he's at least interesting enough to keep the story moving. I'm realizing that this is one of the challenges of writing fiction: As the author, I know that the characters around him either don't know he's a dangerously self-centered creep, or are choosing for their own reasons not to notice. But they don't know. I feel like a spectator, wondering how the story will unfold, even though I've already laid out the major plot points. And every word I write about this guy gives me the shivers.
This is where you come in: I need inspiration for how to do this well! Can you think of books or movies you really love where one character is just awful? Where even the seemingly good things s/he might say or do are tinged with ulterior motive?