Not to take this too far into the realm of technical nerdom:
There is another class of beings: Angels, Demons, Human, and Nephalem the first humans on Sanctuary - offspring of Angels and Demons...Most likely the players characters in the series which is to explain their demi-god strength when fighting the hoards of demons and Prime Evils.
The corrupt rogues are not undead. They are demon class. They were humans corrupted and turned by Andariel.
It is implied that the 3 heros from the first game fall:
The Warrior - The Dark Wander
The Rogue - Blood Raven
The Sorcerer - The Summoner
The Zakarum in Act III are a great example of corruption in the game.
There is also indication of Angels falling to serve the Prime Evils.
Then there is possession:
The Prime Evils possess the bodies of humans on Sanctuary in order to gain a physical form on that plane.
Prince Aldaric and The Dark Wanderer were corrupted and eventually consumed by Diablo...Same as Tal Rasha to Baal.
Beyond that its all fun and games.
It is heavily implied in the game and (the fact its Blizzard who like Super Evil and corruption concepts) that anyone can fall or be corrupted by evil. The lucky ones are just driven mad.
Keep in mind in the previous two games beating the game is always a split victory. Yeah they kill off Diablo but they never seem to do it right. The broken Soulstone forced into the Warriors skull (!!!), the corruption and the destruction of the Worldstone...
In my opinion anything goes here. If a Prime or Lessor Evil wants to twist someone into a goatman I don't think they'd have much trouble. I don't think what Amahain is doing here is rewritting the origins of these creatures just using the fact we are familiar with these things and given some TF fodder.
There is a pretty expansive history in the game and lots of stuff that can be added as the story moves beyond the Monastery.
I really like messing around with the hero classes as No-Man did with his D2 story
http://www.process-productions.com/f...ghlight=Diablo
D3 adds some additional female characters...technically all the characters can be female but it is implied the Wizard and Demon Hunter are canonically female...There is always Leah and Adria...
I am dusting off some of my D2 stuff and working toward some new D3 pics. Might play around with some of the scenes from Broken Sanctuary.