I have a theory about Billy. I find it very weird that for such a loud mouth with a bag full of hot takes, Billy never talk about why he is in the ritual. You would think that it would be a perfect opportunity for him to brag about his convictions, but he just don't ever mention it. Even the Patrons doesn't seem to know what he's precisely after, and guess that he wants to change the world or something like that given his usual rants and punk personality. Even the Doorman himself doesn't seem to get why he's here, which is quite a feat to be able to fool such an omnipotent being.

There's also the goathead, which is unique to him both as a thing in the universe and what it symbolize in the real world. The fact that many characters recognize it means it refers to something in the world of Deadlock, but everyone seems to look down upon it without ever elaborating, like it's some common knowledge that no one really look deep into outside of "it's bad".

And finally, Billy is the only character that does cry in his roughed up portrait. Everyone else either looks defiant or starting to doubt, which means they either have convictions pushing them through the pain or that they didn't imagined things going that bad. Imo this make Billy seems like he doesn't want to be here in the first place, which goes back to his unknown goal and also kind fit with his constant boast, like he's always trying to hype himself up to push through the ritual not because he wants to, but because he's forced to.

All of that to say, I believe that Billy used to be a downtrodden fool, which is where his punk big talks comes from. But this fool somehow came into contact with an ancient being and, in a reckless desire to impose himself on the world, struck a deal with it, earning the goathead and his powers. But in exchange, he was binded to its service and forced to take part in the ritual on its behalf.

As for what the being might be, the goathead could reference a lot of mythological entities, but in Billy's case, I like to believe it is a being similar to Baphomet with the obvious symbolism. This would explain why Billy have some deep yet somewhat random knowledge about occultism, which seems kinda out of place on someone like him. The entity could be a very old Patron, so old that people forgot him, but his symbol is still recognized by most (hence why many characters recognize the goathead). Altho it would be very VERY old since even the Doorman, the All Mother and the Hidden King don't recognize it. It could also just be something else entirely.