I had a friend who was a promising playwright. She had a good ear, could set out a serviceable plot, and she always knew how to keep her stories within the limitations of low-budget theatres. Her problem was that her plays were always extremely lacklustre. They would just lie there like a mackerel on a fishmonger's bench and had about as much dramatic appeal.
The reason? It stemmed from her desire to be a playwright of ideas. She saw theatre as an agent of social change (always a mistake, in my opinion) and wanted to tackle great issues as her characters struggled to find the Truth. Many of her plots were partly autobiographical, which is actually a very good source for material, and those of us who knew her could see where she was reliving old arguments and she didn't feel she'd won the first time around. All well and good. Her problem was that on the one side she had the characters who stood for her point of view and on the other were characters who stood for the other point of view. Her characters were passionate, polished, and articulate. The others were thin, cliché-bound, and crude. The result was that her plays came off as very one sided and were less stories than stacked decks.
The reason is fairly obvious: She didn't try to see her villain's point of view. To her mind, they were wrong and that was all there was to it. She had forgotten that from the villain's perspective what he believes is perfectly sound. It may seem appalling to us, but to him we're the idiots who don't get the point. True, you may not agree with his politics, his attitude toward women, or his views about eating babies for breakfast, but when were you ever expected to make your villain a respectable citizen? He's the villain. He's the one of your major protagonists. If he doesn't come across as a fully-formed character, you have a straw man–in which case, your story is less believable than a panto. Indeed, it's often been argued that the villain should not be just as well drawn as your heroes, he should be better drawn. Villains, after all, get the best lines. They're the ones who get to say "No, Mr. Bond. I expect you to die!"
Look at a good villain such as Goldfinger or Mr Gutman from The Maltese Falcon. They make the novels work. They carry the plot along. Goldfinger is resourceful, cunning, extremely organised and believes sincerely that he can and deserves to rob the world blind. Gutman is dedicated, ruthless, good humoured, sincere, and quick witted. Of course he deserves to own that fabulous treasure whatever the cost. They're monsters, but they're interesting monsters. Or take the example of take the example of General Scott, who arranges a military coup of the United States in Seven Days in May. He isn't a black-hearted megalomaniac with a lust for power. He really believes that he's doing what is right to protect his country against a weak president. It doesn't make him any the less evil, but it does make the cut and thrust of politics and ideas at the centre of the story viable and interesting.
And I'm not just talking about grand villainy here. If you're writing a kitchen sink drama about a woman who is alienated from her mother, don't just see the tyrannical mother from the sensitive, suffering daughter's point of view. See the weakling daughter from the self-reliant mother's side. You don't have to agree with your villains, but it is unavoidable that you must understand them.