Bad guys come up a lot in romance novels. If you're writing Romantic Suspense or something else that requires a bad guy, without a villain, it's pretty pointless, yes? What makes a good villain? The fact the villain can be good, or at least has some good in him.
Villains are bad to the bone, right? But there has to be something inside them that's not bad. Something real, tangible, for the reader to hold on to, some hope of his redemption. There is the conflict with a great bad guy.
When we read a newspaper article about some senseless act of violence, our natural reaction is to ask, "Why?" isn't it?
Google is full of lovable villains: The Joker, Hans Gruber, Keyser Soze, both of the Terminators (those confused looks are just precious, aren't they?)
Look at Lord Voldemort. A quintessential bad guy, but we love him. Why? There's something there to like. His background and the reason he is evil, the fact he's a half-blood, even though one of his main goals is to have a pure race of wizards. And let's not forget he's played by Ralph Feinnes. He is a fleshed out character in and of his own right, in JK Rowling's fascinatingly backwards way of telling his story.
Another villain we love to hate is Freddie Krueger. Okay, not everybody loves him, but die-hard horror fans usually have a soft spot for this burned, witty, misunderstood slasher. Born to a victim of rape (his mother, a nurse in a lunatic asylum having survived overnight in a room filled with maniacs, who brutalized her for hours), bullied as a child, he tried to have a normal life with a wife and child of his own. But ended up succumbing to his own illness and turned to torturing and killing children himself. Now, most of this is told after the first movie, where he found his initial fame through his scarred visage and interesting implements of fear and torture, the knife fingers. But he could have been a normal guy, right? If it hadn't been for the bullying, the circumstances of his conception, or his mental illness. By the way, if you haven't noticed, most of these slasher/horror bad guys have a backstory that comes out sometime in the second movie, so the viewer can hold on to something to make them like the bad guy.
Okay, that was far out there. Let's do Dracula, instead. He seduces his victims with his charm, he has the ability to live forever (who the hell would really want that, that in and of itself is pitiful), and his survival depends on the death of others. Not that it makes him likeable per se, but it does give keep one from hating him totally. He's likeable in his charm and the circumstances which make him play into the victim profile as much as the villain.
So, what's all this about? Great bad guys have a reason to be bad. Not that they don't deserve the stake to the heart or whatever, but there's an arc to them, a resignation to their fate that comes about with their story. Someone once said every bad guy is the hero of his own story. So what's your bad guy's story?