Villain Flaws

Like the heroes, the villains should have well-developed flaws, often ones which will lead to their downfall. Unfortunately, the usual go-to flaw is 'insanity' or some variant thereof, which is used as both the explanation the villain's consistently contrived bad decisions and the motive behind their empty 'mwahahaha!' style evildoing. Now, setting aside the huge problem of conflating 'insane' with 'evil and dangerous', it's simply not an interesting motive. Furthermore, it gives the hero a cheap method for defeating the antagonist-- they merely have to make use of the villain's illogical decisions, or take advantage of the fact the villain has zero grip on reality.

A true character flaw, however, presents both motive and challenges for the protagonist. It's even better if the flaw is a trait which could have been a force for good in other circumstances. The antagonist is now human, and is also free to be as competent as possible. Instead of waiting for the villain to trip over their own feet, the hero has to not only discover the antagonist's shortfalls, but be clever and innovative enough to exploit them.