William is in a band (The Fallen) that Cale likes. In this au Cale steals money from his victims (if he's still a killer) so he can go to concerts. Cale ends up killing groupies that get too close to William. Everyone speculates on William's sexuality. He sees Cale in the audience at several concerts and gives him something at one of them. Media goes into a frenzy when William continues to give things to Cale.
William goes into an alley to get some alone time while the band is on break at a concert and catches Cale killing someone. Cale freaks out because he's worried William's gonna think he's a freak. William doesn't say anything about what he saw; he just smirks and goes back to the band. The next thing he gives Cale (guitar pick?) has a note with it and a backstage pass. Cale knowing that going is putting his secret in big danger and he shouldn't go but ends up going anyways trying to come up with a billion ways to talk his way out, convince William what he saw wasn't what he saw.
Everywhere that The Fallen has played there was a few murders…. Pretty obvious who is responsible to us. Some of the murders Cale looks at and is like wait, I didn't do this??? Cale becomes worried that there's another stalker after William. Cale finds the address to William's main home and proceeds to break in. William has a bunch of demons there to drain dry. This William drinks blood from regular demons, which gives him his power up. Cale is still cambion. Cale kills the demons.
So, William is not happy, but he usually drains a demon before moving on to his next. Cambion blood would be really addictive for him if he were to consume any.
He doesn't know that Cale's a cambion yet; just thinks that he's a lucky homicidal fan.