I mentioned it in another thread, but right now I'm really into this comic called Irredeemable. The premise is basically what would happen when a superhero with Superman level power (and I'm talking silver age power) and no obvious weaknesses is simply not emotionally equipped for the job of being the world's hero. He's can't handle the constant cries for help his super-hearing picks up all the time, no matter where he is. He can't handle being criticized by people who are ungrateful to him for his rescues. Most of all, he can't handle being betrayed by the people he's been protecting all this time.
When it all becomes too much, he snaps and decides humanity isn't worth saving anymore. In fact, he decides that he was wrong to be saving such a worthless species all this time, and turns into the most heinous global mass murderer in history. The general story of the comic is split along two threads. One involves a team of lesser heroes (and even a few villains who know that the earth being destroyed would be bad) trying to figure out The Plutonian's origins, powers, and especially weaknesses so he can be defeated. The other is from the point of view of The Plutonian himself, and delves into just what kind of events could turn a Superman into a Darkseid.
It's a fascinating comic, and I highly, highly recommend it.