Kull went completely mad making the soul stone, and wished to put the souls of angles and demon into his body, can you image the monster he would have become if he had achieved this feat.
Second every person you met in the game says that, he was mad crazy insane evil, his brethren called him a murderer, torturer, monster.
They killed him cut him into pieces and drained his blood, then hid his body in another ream, do you think that they did his for kicks and giggles.
He wanted to become a god and rule over the universe, also I think he will get his hand on the black soul stone and be the next expansion villain.
The fight did not start because he know about act three, the fight started because, it was obvious that he wanted to use the demonic power now in the soul stone to become a dark and possible unstoppable demigod.