For once, the Explainer piece on Slate refers to something of serious curiosity: the significance of beheading.  My suspicion had generally bounced between a twist on “the eyes are the window to the soul” (thus, supposedly being able to watch it leave the body when you chop off the whole head) and simple expression of rage.  Considering the tendency towards violence of humanity, I wouldn’t be surprised if there were some past civilization/army/group of random nutjobs that cut off a limb & beat the person to death with it like in Mortal Kombat.  Shit, Midway had to get the idea somewhere…