Yes, Zul'jin was a good guy in Warcraft II, but He became evil when blood elves went to the horde. Zul'jin hated the Elves more than anything thing else. So he turned against the Horde once this happened. Vol'jin is just a lame version of Zul'jin
The Amani trolls have always been evil and bloodthirsty, and Zul'jin more than any of them. He only joined the Horde during the second war cos they were killing elves.
Zul’jin is the last boss of Zul’aman Instance. He drops 133 & 138 level items. Zul’jin will be able to draw upon the ghostly spirits of each of the four liberated animal gods from the instance and change his own shape to assume their powers. Every 20% of his health, he switches form.