ita, the werewolf/vampire/shapeshifter souls are human souls before they're infected

Where is this stated?

That losing of one's former self in a slow (or not so slow) transformation into something other

Where is this slow transformation into the other? Where did we see this?

I've been making some guesses because I don't see the canon out there, but it seems I've missed some important stuff.

I guess I see Sam's situation as pretty clear--he had a soul tortured specifically by Lucifer in Hell. Castiel assumed the results of that specific damage onto himself. We've never seen anything like that with monsters in Purgatory, so I'm not sure how we can draw parallels between anything that's gone before in a different location subject to different hands and this, even if souls are transformed (no consistent evidence across monsters--I could see a werewolf argument, since they're perfectly normal most of the time, but why vampires? Where'd that come from?) that they are being tormented or are in any way suffering.

I mean, I wouldn't even think Cas would go mad from doing the same thing to Dean or any other person who'd suffered in Hell, just Sam.

Maybe that's the difference. I see Sam as an exceptional case, and you see him as one of many.

I'm just not seeing the evidence of monsters in anguish that you see as so obvious. We've had tormented monsters called out as exceptions to heighten a handful of stories, but most often it's monsters both Dean and Sam would have no hesitation putting an end to, Mr. Black and White, and Mr. 1 Million Shades of Grey themselves.

Also, did we have any evidence that Eve was connected to the monsters before she came topside? I get what you mean by hive mind, although it's not a term I'd have used. She seemed to be broadcasting to them, and reading their senses, but when I think of hive mind I think shared consciousness, and I didn't get that here.

Where is this stated?

I think the alpha vampire told them -- he was asking them where they thought someone like him would go when he died.

But he doesn't say human souls are changed into monster souls. He just says humans go to Heaven or Hell, and that monsters go to Purgatory. Here's the transcript.

Oh, right. Never mind.

So when human is turned vampire or werewolf (stuff we've both seen) their soul does not end up in purgatory? Cause I thought that was the store the mother-of-all was telling.

I've never seen that said. What we've been told, and as much as I could confirm on re-reading, is that dead vampires and dead werewolves go to purgatory. As far as I can see, the question of what happens to me if I get vamped or wolved is unanswered. I can't find canon on if I am changed or replaced. I'd suspect that in a werewolf situation, it would be changed, just because you're so you for so much of the time. But with vamping it could go any direction at all.

It was certainly how I took the conversation with her in "Mommy Dearest." She talked about taking all the souls and leaving none for Crowley to receive in the future - that doesn't gibe with human souls going on to their regular reward/punishment when bitten.

Note too that Dean was turned into a vampire in "Live Free or Twihard," and the method used to turn him back didn't seem to involve calling a missing human soul back from the great beyond. The way Sam walking around without benefit of his soul was such a surprise to everyone seems to imply that it was kind of unprecedented, and souls normally stay with their bodies until death.

Also, did we have any evidence that Eve was connected to the monsters before she came topside?

The various monster species were all stirred up and beginning their recruitment drives well before she was summoned back by the dragons. I assume that was her doing in response to Crowley torturing her offsping for info behind the scenes.

My assumpiton was not that your soul instantly goes to purgatory when vamped or wolved, but that if you are turned into a vampire or werewolf you go to purgatory when you die. Othewise how does turning people into monsters win mother-of-all souls?

I'm just not seeing the evidence of monsters in anguish that you see as so obvious. We've had tormented monsters called out as exceptions to heighten a handful of stories, but most often it's monsters both Dean and Sam would have no hesitation putting an end to, Mr. Black and White, and Mr. 1 Million Shades of Grey themselves.

I'm not necessarily certain the dead monsters are all in a state of torment (though it wouldn't surprise me, what with apparently floating forever in a dark void with Mother-of-All and the Leviathans for company). But I would think that being linked to them and their inhuman urges and such would be destabilizing for a non-monster consciousness such as Cas' (or anybody else's).

For those who already know some Glee casting spoilers, see this item and its reference to Sam and Dean. Dude, Wincest frigging precedes us everywhere.