He's certainly a lot smarter than every other character in the game, including the player character.
He calls everything right.
PC allies not trustworthy - check
Angels are bastards - check
Make humans all super-powered - check
Try and ally with us to fix everything - check
And then we kill him because we won't listen to him.
Zoltan Kulle was a good guy, no matter how many times Blizzard makes him laugh like Snidely Whiplash.
Killing him led directly the re-creation of the Prime Evil, who almost certainly isn't dead, and the loss of the Black Soul Stone, the only weapon useable against the Daemons and Angels.
So damn stupid.