You have REALLY watched too many movies.
With my idea, I was really considering the fact that the player character represents someone of unusually effective knowledge of how to cause destruction with their weapon. I expect the he/she will understand how to synergize the movements of their body to multiply the impact force to awe-inspiringly high levels. No, I don't expect the normal swing of a mace to lop someone's head off.
Though, now that you've made me reconsider, it does seem much more likely that the body would fly away from you before the head separated. And even more likely that the head would explode before simply leaving the body. Good point, maybe I have seen too many movies, lol.