Hey, everyone who complained about the Scourge world event being too wussy -- the Burning Legion sends its regards. Eliah Hecht has documented that Highlord Kruul is owning everything on most servers, but no one has answered the question: Is it possible to kill this guy?
Short answer: Yes. He appears to be a renamed Kazzak with most of Kazzak's abilities, plus he has Kazzak's loot table. He seems to hit harder than Kazzak and enrage sooner, but an organized, or even slightly disorganized, raid of 40 people can kill him. WoWWiki has a good strategy guide for Kazzak. Exo and Trans killed him on Smolderthorn, as did groups on Illidan, Medivh and Kalecgos, apparently.
Long answer: Probably not, at least if your server's anything like mine. Kruul heals massively every time he kills a player or pet. So if Kruul spawns near a major city, it's nearly impossible due to the number of deaths. Unless you can clear the area of everything but organized 60s working in a raid group, just get naked, sit back and watch the carnage.