Lots of things. There's no formula for making an antagonist. Really, you shouldn't focus on making them good villains, but good characters. You can make your bad guy someone the reader or veiwer can identify with, someone representing some big, real world danger or evil, someone who is a victim of their own darkness, or even someone who isn't all that bad, but just happens to be on the other side of the arguement. Doesn't matter. You just have to make your bad guy more than a two dimensional plot device. He or she has to be a fully realized, three dimensional character with thoughts and feelings and morals and beliefs and interests and flaws and strengths and weaknesses that the audience, for whatever reason, cares about.