Zoltan Kulle was one of the Horadrim, who actually knew of the wars between angels and demons that was a secret to most denizens of Sanctuary - and presumably, they (or at least Kulle) also knew the origins of Sanctuary, that everyone on it is a Nephalem. Basically, even the whiny mayor of Tristam is a Nephalem, except he hasn't awakened his powers or tried to fight the legions of the Burning Hells.
Based on his actions - following through on the promise to give the Black Soulstone, making it in the first place for the sake of humans having to no longer fear angels or demons, and warning the player that he is being used ... I personally think he's just misunderstood and is far more on your side than most of the other NPC in the cast. I feel bad having to kill him (over & over, for each character on each difficulty... hehe)