Furthermore, if you draw the graph the way Neel seems to suggest, then the bodyguard is adding the antidote without dependence on the actions of the assassin, and so there is no longer any reason to call one "assassin" and the other "bodyguard", or one "poison" and the other "antidote". The bodyguard in that model is trying to kill the king as much as the assassin is, and the assassin's timely intervention saved the king as much as the bodyguard's.