I don't like this idea. A Holy/Unholy, Good/Evil or Light/Dark based class sounds bad in my opnion.
You see - if a holy warrior would fall in corruption, surely he would loose their holy powers, once paladins, priests, clerics, both good and evil, do not have power from himself, it's just a favor of a deity. In D&D things work like that but even more restrict (if you are a monk, wich must be lawful, and start to do lots of chaotic acts you will soon became a neutral character and loose partially your monk's skills. Partially because a monk in D&D are not a 100% divine class like clerics and paladins).
If this class was some type of arcane character it would make sense to me, but a class who are linked to divinity could never use those two oposites powers, unless if this class follows a neutral deity that praises balance, but in diablo we have no clue of such thing once theres only Heaven (order) and Hell (chaos).
There won't be any evil classes because this is a game of good vs evil. The evil lives in hell and the good is trying to stop hell. It's really that simple.
None of the previous Diablo classes have been evil. The necromancer was not evil -- he used necromantic magic to do good. You won't see any classes with skills like, "Devil Pact" or "Sell Your Soul" because that would make you on Diablo's side.
I don't think this is truth anymore. The large content of lore released makes me think that it's a simple battle of evil vs. good. Theres a huge possibility of Imperius be one of the villans, so the high heavens (good) may be our enemy.
For me diablo is about the struggle of humanity (wich means something betwen good and evil, like a balance) to survive against the influence of absolute Chaos (burning hell) and maybe now absolute order (Hihg Heaven).
A proof of that is the fact that in D2 we played allmost evil characters (necro) and neutral (barbarian, assassin)... So how can be all about a good vs. evil battle if i play bad guy?