So let's run this through. What are the scenarios for scum!Joseph?
1. He actually has the role he claims (or some variant thereof)
First let me state that I think this unlikely because giving scum an extra kill is quite impossible to balance. But okay, let's for a moment work under the assumption that this is possible. What does he do last night? He targets Xerxes with it. I mean, it's conceivable that he lied about this, but really why? With roles confirming stuff, this is dangerous. So we'd have to assume he tells the truth about targeting Xerxes. This makes little sense for scum. Scum targets the player they attack? Why? It robs them off a night kill - and it doesn't even look good on them.
This case is hardly possible. Occam's razor tells us to dismiss it.
2. He lied about this role.
Okay. So he might have another role. This is something we can make sure I can uncover tonight, with little possibility of me dying. But I doubt even this is the case. I mean, what sort of super-crappy fakeclaim is this? If he lied, why claim to have targeted Xerxes? He did the night attack, and is afraid of being Tracked/Watched? There's no such role. Why claim something that in order to work needs help of some other player? Maybe to insinuate that someone is lying. But if this is the case, then why do a full turn before it even plays out? Sorry, but I don't really see this either.
Okay, so there's this chance that he has a role Poisoner that does something different. This would be extremely misleading, like close-to-bastard misleading. It's convention here that mafiascum names are mafiascum roles, and breaking that should at least be announced pregame. And Arch knows about Posioning, he used it in RMM11 I think.
So there's no conceivable scenario where Joseph is scum for me. Still, if you want to be really sure, I can check him tonight. But why would we lynch him? Makes zero sense to me.
As for responding to your "arguments" I assume you mean this big pile of misdirection right here.
I've already address #1 -- he has a killing role, it's called MAFIA.
As for #2, is it worth letting scum to live for another day and kill another night just so you can get his role name? Poisoner is a scum name, after all, and you are playing the "mafiascum names are sacrosanct" card, so Poisoner should be scum (or SK). But even if this was normal, and you get "Roleblocker" as his name, then woo, you've DONE NOTHING BUT LET HIM KILL PEOPLE and LET SCUM LIVE.
Why would we do that? Why? On the off-chance that he is a Poisoner, at which point we still don't know his alignment?