Two issues here.
1) It's possible, though not certain, that the head's supposed to take double damage.  That's not unknown in location-based HP.  Worth discussing that, IMO.

2) the if-statement there is so you don't take overkill damage on a fatal hit.  hp_head <= 0 => Game Over, press spacebar.  The game looks out for that enough to make me think it's worth keeping around.