What is Angel Alastor’s backstory?
I assume you're referring to the Undercover Angel AU
I actually went into his backstory a little bit in -> this ask <-
Alastor was still a serial killer (and he'd killed with just as much gritty delight as in the show) but he fell under a loophole Heaven had made for Adam (to excuse why Adam and his army got to kill thousands of human souls each year without repercussions).
Heaven is aware that Alastor was a serial killer, but like the show said, they don't decide who gets into Heaven. Winners just show up, signifying that they'd passed divine judgement and earned their way to salvation.
Thus, he was judged a Winner, despite his very obvious sins. Alastor knows he shouldn't be there. Heaven knows he shouldn't be there. But they're not going to cast him to Hell because, technically, he did pass divine judgement. Nor are they going to reverse the loophole they'd made for Adam, because that would mean they have to also cast Adam into Hell and they can't have THE First Man of humanity down there. That's not very good for their image. (They'd also have to explain why Adam was suddenly cast from Hell, which mean they'd have to reveal the Exterminations, which would not go over well as most beings in Heaven didn't even know about it).
So, Alastor got to stay in Heaven. Their solution was just keeping an eye on him, having him keep a low profile, and deciding to never, ever, ever talk about it ever again.
Essentially, they brushed it under the rug and are pretending that everything is hunky dory dandy because Heaven is never wrong.
They don't make mistakes.
Ever.
