[dogma1138]

Doesn't have to be that extreme in many cases it can be just a concussion which leads to intracranial swelling you feel "fine" till you go to bed and never wake up, other common side effects of getting repeatably punched in the face also include things like an increase likelihood of having a stroke due to blood cloths forming within cranial blood vessels (Circle of Willis, Cerebral and Lenticulostriate Arteries).

Killing a man with your fists isn't that hard, much training in various full contact martial arts actually goes into practicing how not to kill your opponent during a fight.