Okay I think where Vox went wrong with his offer to team up with Alastor is the wording.
I bet that if Vox just said that he wanted to make a deal, Alastor wouldn't have been so harsh. Because every one of Alastor's relationships we've seen so far has been transactional.
Rosie owns his soul and is using him for something. Mimzy only goes to him when she needs protection. And he owns Niffty's soul so she's pretty much stuck with him.
And I imagine he also felt "used" when he was alive what with him being a POC. I imagine his white bosses just begrudgingly hired him like "Okay fine if we can get people to listen to our station more, I guess we'll put you on. Especially since no one will ever see that you're a POC". I bet that line he said in s1, "this face was made for radio", was actually said to him when he was alive.
The only dynamics Alastor has is either him being taken advantage of or him taking advantage of other people.
So, when Vox was asking him to be partners and saying they were friends, I think Alastor didn't believe he was being genuine. Alastor probably thought Vox was trying to trick him and soften him up so he could take advantage of him later down the line. Because wtf would anyone be friends with him?
So I feel like if Vox just said he wanted to make a deal, sure Alastor probably still wouldn't have accepted it but he would still keep his respect for Vox. It'd be like, "yeah this guy just wants to use me but at least he's making his intentions clear".
Plus there's also the fact that Vox had a crush on Alastor (which Viv did confirm today on Bluesky). Obviously we don't know if Alastor picked up on that, but just with the way Vox was acting in this scene alone it seems his crush was pretty fucking obvious.
So you take that and combine it with Alastor being aroace, his misandry, and his refusal to believe that anyone could genuinely care about him... yeah it's no wonder he was so harsh.
I said a while ago that if Vox did ever confess his feelings to Alastor, Alastor would probably be like "Okay so you only became my acquaintance so you could eventually have sex with me".
And I imagine something maybe something similar happened here in Alastor's mind. When Vox said they were friends, Alastor probably thought "Oh God that's how it always starts. First it's 'friendship' but then that's never enough and they beg for something more. And hell with the way Vox is acting like a schoolgirl with a crush that's definitely bound to happen."