The neck is full of sinew and muscles going different directions, and it is surprisingly hard to literally decapitate someone without a heavy, swinging blade and a firm anvil or block to react the blow. A sharp straight edge blade should cut through steadily, but something the size of a ka-bar type knife just isn’t going to lop right through. A scalloped edge bread knife really isn’t going to be a lot more effective; for efficient sawing, you’d really want a saw, something like a coarse-toothed hacksaw or a large dressing saw, similar to what is used for field amputation. In general, this is just a really difficult way to kill someone compared to normal stabbing of the chest, through the back of the head, kidney, shooting in the head or chest, hanging, et cetera.
What interests me is the psychology behind beheading. Certainly the brutality is manifest–save for the guillotine, beheading is a messy, error-prone, and typically inefficient method of execution which can be practiced effectively only by true sociopaths–and the resultant fear that can be induced is an effective means of cowing a subject population, hence its popularity by bloody despots throughout history. On a personal level, though, it also detaches the the cognitive apparatus, vocal equipment, and/or “windows to the soul” from the rest of the body, guaranteeing that the subject will not rise from the dead, or at least, if he does so, he’s going to have a hard time navigating. It seems to me that there is a great deal of fear behind the act of decapitation practiced by someone who is not a congenital psychopath, and that participating in a decapitation is really a way of binding the participants together in a horror pact.