"If ... two men attack another without any intention to cause death or grievous bodily harm, and during the course of the attack one man forms an intention to kill the victim, and strikes the fatal blow with that intention, he may be convicted of murder while the other participant in the plan may be convicted of manslaughter. The reason why the principal assailant is guilty of murder and the other participant only of manslaughter in such a case is that the former had an actual intention to kill whereas the latter never intended that death or grievous bodily harm be caused to the victim, and if there had not been a departure from the common purpose the death of the victim would have rendered the two participants guilty of manslaughter only. In some cases the inactive participant in the common design may escape liability either for murder or manslaughter. If the principal assailant has gone completely beyond the scope of the common design, and for example 'has used a weapon and acted in a way which no party to that common design could suspect', the inactive participant is not guilty of either murder or manslaughter. If however the use of the weapon, even if its existence was unknown to the other party, is rightly regarded as no more than an unexpected incident in carrying out the common design the inactive participant may be convicted of manslaughter."[7]