he tried to brainwash him when Alastor declined

Perhaps that's where Alastor's aversion to screens stems from. Of course, he prefers his era of technology, but I believe it's more than just a preference. I could be wrong. But it feels like there's more to it than Alastor just not liking new tech

I think the two were genuine friends at some point. Or just had a better working relationship before their fight. Alastor, before leaving, had a big role in the entertainment district, as we know. So I think they worked together, and when Vox tried to get Alastor to join him and the other Vees,

Owning each other's souls bc that's who they belong to.

(I'm not sure two sinners can own each other's souls, but this is fiction and a simply headcanon 🤷🏽‍♀️)