Zul'Jin was a hero of the Trolls.
When the Alliance stormed the land that the Forest Trolls were on, the Blood Elves were with them. They took him, beaten him, and cut his eye out. Then, when he was imprisoned, he cut is own arm off to escape. Then, he realized that the Blood Elves were with the Horde. Zul'Jin was allies to the Horde, until he saw that the Blood Elves were allied with the Horde. So, then he hated the Horde and the Alliance. Therefore, that is why Alliance and Horde can kill him.