Building off of your spoiler,

I’m guessing there is something a little bit more to Billy that we still don’t know, specifically as it relates to his spirit inhabiting the now dead William Kaplan.

First at the end of the episode Agatha mentions something about Billy breaking the rules in terms of him having a body.

We’ve actually seen this before in Multiverse of Madness when 616 Strange inhabits the body of dead Strange. Demons appear and terrorize Strange.

So a spirit entering a dead body breaks someone’s rule. However Billy is clearly fine. In which case Billy, or someone on behalf of Billy, made a deal to keep Billy alive.

Now in this episode we saw Lilia put the sigil on William Kaplan, before knowing its purpose. If she has this power, it’s possible others also have this power.

So someone, knowing it would payoff later, facilitates Billy Maximoff’s spirit entering William Kaplan. In fact this same someone probably also ensured the accident happened the way it did.

So who is this someone? Well in Multiverse of Madness we see minor imps or demons. We also visit the throne of Wanda/Scarlett Witch, created by the demon Chthon, author of the Darkhold. But I suspect there is probably someone higher up in the demon tier list. Or maybe Chthon goes by multiple names. A name we’ve already heard once in this series. Someone Agatha is known to have made a deal with.

That’s right, Mephisto.

So Mephisto saved Billy, knowing it would pay off later, likely to control Wanda. But also it could be an Ego the Planet situation where having Father/Son, or in this case Mother/Son makes everything more powerful.