Really Jon? What about that tubular shaped thing that holds your head up that contains the trachea, and the jugular and carotid arteries, that sends an oxygenated blood supply to the brain from the lungs and heart called the neck? Very rarely protected and easily slit. Even a small razor blade can sever anything in it.
Lets not forget the eyes. As for blade length,you only have to go in 3",give or take, to reach the off button. Or, one, good, deep slash across the eye brows will send most muggers on there way. Head wounds really bleed,so if they want to hang around it won't be with 20/20 vision. Now having said that,just shoot 'em!