People said the same thing about Illidan when we found out that he was going to head up the Black Temple (and for a while, Blizzard pushed the rumor that we might not kill him, but we eventually did, us murdering bastards). Zul'jin's roots go all the way back to Warcraft II, when he united with the Horde against the incursions of the Human Alliance. Even now, trolls (both NPC and some players alike) revere him as "Great Chief."
There is one big difference between Zul'jin and Illidan, however, and that's that Zul'jin is technically already dead.. kind of. You could only beat a mission in Warcraft II by killing him, and trolls everywhere ask for "Vengeance for Zul'jin," which implies that he's been killed. But a captured Hakkari troll from Zul'Gurub says Zul'jin is "probably dead," and even after the Warcraft II mission (in which he was supposedly killed) Zul'jin apparently was seen in the Alliance internment camps. So even if we do kill Zul'jin in Zul'Aman, it likely won't be the first time we've done it.
Oh, and there's one more catch. WoWWiki says, under Speculation, that "Zul'jin," according to the WoW RPG, is actually a title, not a name. So there's a chance that the Zul'jin, head of the forest trolls in Zul'Aman, isn't even the same guy that allied with Doomhammer against the Alliance.