There could be various reasons. Maybe he had mental health problems, or there was a major conflict within the family that pushed him over the edge. Whether it's true or not, it's a disturbing thought.
It could be Dumbledore from beyond the grave. He might find a way to reach out and summon Harry, believing that only Harry has the potential to truly defeat Voldemort in this new, dire situation.
Harry might use his wits and the environment around him while in captivity. For example, if he's held in a place with magical artifacts, he could use them to his advantage. He could break the chains that bind him with a spell - enhanced object and then make his way to Voldemort and defeat him.
Well, it could be for power. Maybe Voldemort realizes that by saving Harry, he can gain some sort of power over him or use him as a tool in a more elaborate plan.
Perhaps it was because of the power he amassed in the process of defeating Voldemort. In the fanfic world, power can be a very scary thing to those in authority. Harry's new - found and possibly overwhelming power could have made the existing magical power structure feel threatened. So, they arrested him as a way to control and contain that power, rather than seeing him as a hero who saved the wizarding world.
No, Joker wasn't the one who killed Batman's parents. The culprit was a common criminal during a mugging incident. However, Joker has often taunted and tormented Batman about this tragic event in their confrontations.