!! So, what actually kills someone, when they're headshot?
In a nutshell, either damage to the autonomous centers of the brain itself, sufficient to disrupt its operation; or blood loss. A headshot will usually cause serious bleeding, but there are other places to aim at that are more likely to be effective if you're counting on blood loss to do the job.
There are three mechanisms at work on the brain in a headshot: Laceration and crushing; cavitation; and shock waves. The first, laceration and crushing is the damage caused by the physical movement of the bullet (and possibly bone fragments) through the brain tissue. How much damage this causes is dependent largely on the ''caliber'' of the bullet. Clearly, a bigger bullet will directly affect a larger area of the brain.