In any case, what Osama did would clearly be an act of war if he had been a general acting on the orders of a government. So what makes it not an act of war just because he's acting on behalf of an entity that isn't a sovereign state? Surely, it is most logical to measure the justifiable response to an action in terms of the nature of the action itself, not the political affiliation of the actor.
Finally, as you note, it's hard to say that there is any rule of law in Pakistan. Laws only have meaning to the extent courts have jurisdiction to enforce them. "Murder" isn't a universal constant--it's a law defined in the context of some sovereign entity's legislative jurisdiction. Killing a terrorist on the battlefield is not murder if there is no court that has jurisdiction over the actor that would call it murder. Indeed, it seems utterly non-sensical to me to argue that someone like Osama, that has rejected the criminal jurisdiction of any state, can turn around and claim the protection of the laws of that state.