Why did they do it? The answer to this question is almost always motivation. Along the same lines, in fiction your characters should have motivation to do the things that they do. The majority of people don't simply wake up one day and decide to rob a bank with no motivation; your villain shouldn't be part of the minority. There are two types of motivation: internal and external. Internal motivation is found within the character's head. Internal motives include morals, love, loyalty, anger, and many more concepts and emotions. External motivation brought on by physical objects and social opportunities. External motives include things like pending fame and fortune. For each action a character makes, that character should have internal motivation, external motivation, or both. However, if your character is insane, go ahead and ignore everything above.
Have you ever written a character who wasn't mentally sound to keep motivation out of the equation? Leave a comment, I'd like to hear about it.