Zoltun Kulle was a powerful sorcerer specialized in alchemy who was invited by the Horadrims/Tyrael to save Sanctuary from the 3 great evils.
During the hunt, he was entrusted with the custody of the 3 soul stones, and it is said that he developed a certain obsession with them.
After the 3 were captured it is said that he, in his obsession, abandoned the Horadrim and dedicated himself to the creation of an artificial soul stone with which he would empower humanity.
He went crazy… and since then he has been considered an antagonist of the story.
But some things don’t quite add up.
After the fight against Baal and realizing that the fragmented soul stone would not hold the demon for long, Tal Rasha, current leader of the Horadrims, proposed finding another stone, to which Zoltun responded: “Are you mad?”.
Tal Rasha responded that they had to find Tyrael, since he could give them another stone, but we all know how everything turned out.
Tyrael did not give another stone to the Horadrims, and Tal Rasha offered herself as a sacrifice to complete Baal’s imprisonment.
Kulle was perhaps in charge of designing the magical prison due to his specialty with alchemy and Tyrael himself sealed Tal Rasha’s tomb.
It is likely that Zoltun realized from this that the angels had no real interest in humanity, as Tyrael preferred to sacrifice his companion rather than provide another soul stone.
Many would say that Zoltun Kulle didn’t really care about his companions, and therefore, Tal Rasha wouldn’t really have been important to him, however we know that before joining the Horadrim, he worked with another mage. In the notes that we can find from Kulle in reference to his work with this magician, we can see how Kulle knew what companionship was. Zoltun knew what it was like to appreciate someone (and lose it due to a demon).
Therefore, there is a great possibility that Zoltun Kulle abandoned the Horadrim and undertook a project that he himself considered crazy, so as not to experience the loss of a companion again (at the hands of a demon, or an angel).
Therefore, Kulle’s untold story would be how he tried to avoid losing another being important to him… The story of a true hero who achieved the impossible, the madness of creating a soul stone, and failed anyway.