Amount of blunt force to kill a person
TL;DR: It's highly variable. Don't go whacking people over the head.



All of those things are factors, yes; the two most important ones being force and location. There are subsidiary factors, but most of the time, insofar as there is and identifiable proximate cause of death, it boils down to force and location.

There are two primary paths to death when whacking someone over the head: trauma to the brain, and trauma to the spine. Trauma to the brain is caused by significant force applied to the cerebellum; either in one-off high-impact events, or repeated blows; and trauma to the spine is much the same, injuries caused by shear force to (in particular) the cervical spine; again either in one-off high-impact events or repeated blows.

(There are also secondary paths to death, such as blood loss or infections associated with lacerations of the scalp, face or neck.)

Blows to the top of the head, or blows delivered when prone tend to introduce direct damage to the brain, whereas blows to the side and back of the head tend to introduce damage to the spine. Either is fatal, but for different reasons: Cervical spine injuries hamper communication between the central nervous system and vital organs such as the heart and lungs, leading to hypoxia and suffocation, as well as cascade effects with lesser injuries, whereas brain injuries are more directly deleterious and fatal.

All of that being said, however: While cervical spine injuries can be debilitating, they can be ameliorated by muscles associated with the damaged structures. Direct brain injuries aren't nearly as forgiving, but some people do have Simpson Syndrome to varying degrees.

In addition, there are various somewhat common congenital defects that due to the relatively peaceful nature of our day-to-day are never discovered, but can have a significant effect on response to blows to the head. (This is part of why, if you're ever in a car accident, paramedics will be very keen to evaluate you, even if you have no outwards signs of injury.)

Long story short: Punching someone in the head is a bad idea, unless they pose a very direct threat to you; and you mean to incapacitate them; and you don't mind said incapacitation being potentially permanent (to the point of death).