My theory is that Mimzy was once Alastor’s best friend, he maybe even thought he might have been in love with her at one point, in that manner aroace people who don’t know they’re aroace often do, that she was then sacrificed by this cult, and that Alastor had them infiltrated and taken down every member of that cult over decades, losing more and more of himself each time as those he killed grew less and less directly connected to the crime, “sorry you just got in my way, maybe I enjoy it just a little bit does that make me insane”, but that in the act of doing so, that contradiction of righteous intent and pure depravity, along with whatever ritual for power the cult had initially started upon, let’s say in honour of Roo (the root of all evil in the Hellaverse, a character yet to be introduced), and that on Alastor’s eventual death, he then found himself in Hell all-powerful, and set about bringing down those Overlords beneath his power continuing this moral code, until he hit a wall, finding himself having ended all Overlords less powerful than him / otherwise came to his senses on meeting Mimzy again and finding her to have gone to hell, recognising that “weren’t you an old pal of mine” and really nothing more and deciding mere power, entertainment is all he now desires. But that still, out of obligation, that he must kill Lucifer, the Devil, for a sense of completion, while still being allowed to exist himself afterward. Which leads to whatever deal he ended up trapped in seven years before the events of the series, and him getting Charlie to make a deal with him to “harm no one” at a certain point in the future: to ensure that when the time comes that he will be able to kill Lucifer, and the one person able to stop him won’t be able to. To really make Alastor a true Exterminator, more measured, the embodiment of how the road to Hell is paved with good intentions.