We had a campaign in D&D where we assembled a steampunk-ish time machine. After many sessions travelling through time, uncovering mysteries and learning harsh lessons about changing history, we had to stop a time-travelling cult from destroying the gods, and therefore the world. We failed.
Our machine crashed, we were stranded earlier than we had ever been able to travel. We found the Gods, but only a few of them were present- it was as if some had never existed. Then we realised- we had to become those Gods. Our party was entirely divine (Cleric, Paladin, Avenger, Invoker), and each of us was a worshipper of a god who had been unmade- and we were only people in existance with enough knowledge of forgotten deities to assume their roles.
But two of the players were worshippers of Io (in his twin forms of Tiamat and Bahamut, who of course would form later after Io's 'death'), and only one could become Io. The other would have to be the un-created Asmodeus.
So the most just, honourable and dedicated Lawful Good Paladin I've ever seen roleplayed became the god of tyranny and evil. If he hadn't, the gods would never have defeated the primordials, and the world would never have been completed.
In our setting, Asmodeus is every bit the epitome of evil you would expect him to be. Nobody but the gods who abide his presence know him as otherwise. He adheres to his role because he knows he has to- and that in doing so, the world can exist. He can never tell anyone his duty, and noone who knows can ever discuss it.
In the farthest recesses of the Nine Hells, in a chamber sealed tighter than any other in existance is a pocketwatch of finest gnome craft with a photo of his family in it- his wife, son and little baby girl.
They were killed by an orc army marching under the orders and banner of Asmodeus. Their deaths are what drove him to become an adventurer.