Assassination has always been a touchy subject in international law. Officially, assassinating your enemy is a violation of international law, but it all depends on how you define "assassination." In World War II, when the US obtained intelligence that Japanese Admiral Yamamoto would be flying around the Solomon Islands, inspecting troops, a squadron of fighters was dispatched and Yamamoto's plane was shot down. Although Yamamoto was specifically targeted, most commentators don't regard that as an "assassination."
Much depends on whether the target of the attempt is a combatant or non-combatant. Shooting someone who is shooting at you is OK, shooting unarmed civilians is not OK. That's easy. But there's a big grey area between those two extremes. Is the unarmed guy who orders the guy to shoot at you a combatant or a non-combatant? That's not clear. On one hand, if he's a combatant, that would make any decision-maker in a combatant government a legitimate target, which doesn't sound right. On the other hand, if he's not a combatant, then that would mean it would be a war crime to kill Hitler, but not a war crime to kill one of his bodyguards, which doesn't sound quite right either.
Nowadays, we usually avoid the moral ambiguity by simply claiming that we're not aiming at the leader -- but if he steps in the way of the bullet that's not our problem. American and NATO planes have targeted Gaddafi's family compound in Libya on several occasions, but Gaddafi has never officially been a target. That's because everyone suspects that targeting him specifically would be violating international law. But if the Libyan leader just happened to be in the blast radius, well, that's the way the cookie crumbles. I like to refer to that as the "lucky shot defense."
Terrorists, however, occupy a special category. Under traditional principles of international law, terrorists are considered to be like pirates (and foxes
): hostem humani generis
-- enemies of all mankind. They're not considered members of an armed resistance, so they don't enjoy any protection under the Geneva Convention
. As such, they're fair game for everybody. That's not to say that anyone can do anything to a terrorist, but that the limits are certainly more expansive.