Billy grew up foraging in the Wasteland and was eventually hired on to protect caravans. Life was nothing but profit and survival until he came across Maggie. After killing the raiders who had massacred Maggie's parents, he took her under his wing. He brought Maggie to Megaton and helped fend off a Raider attack. He was given the house of a settler who was shot in the attack. There are a few around town who say that Billy was the one behind the bullet that bought him his place in Megaton.
Colin Moriarty, on the other hand, has his own thoughts on Billy Creel. According to what can be found on his terminal, he suspects Billy Creel killed Maggie's parents himself, and kept the girl as a "trophy". Moriarty also describes him as a "Nuka-cola swilling prick", which suggests that Billy doesn't drink alcohol.
"Megaton is the best little town you'll find in the Capital Wasteland. Don't let anyone tell you different. Strong walls, strong people, and strong booze. We've got everything you need to make a life these days."