One thing about characters is that they are neither all good nor all bad. Even the most evil character will show some foible that makes the reader care in a small way, but not for long. I call the exercise, "Taming the Animal" because all protagonists will have some weakness they need to tame--and so will your antagonist. If those weaknesses are left untamed, they often lead to the demise of that character.
For this exercise, take one of your favorite stories and identify the character's weaknesses. Then write a scene where that weakness is no longer tamed and bring the scene to a new conclusion.