In some cases a punch to the back of the head can indeed kill you and sometimes can kill you instantly.
When someone gets punched hard in the back of the head the punch in the back of the head can damage the cervical vertebrae and subsequently the spinal cord, which can possibly lead to serious and irreparable spinal cord injury.
If someone does a rabbit punch to someones back of the head then it can detach the persons brain from the persons brain stem.
If your brain gets detached from the brain stem from a punch then it can kill you instantly.
So punches in the back of the head are extremely dangerous and should not be done at all unless you have no choice to protect yourself from harm from someone else.