well, because the player will gain health as well meaning you'd be in some kind of chamber.
What he needs is for the monster, when shot at, gain health, instead of the opposite..
What if you where to use negative numbers in the Half-Life code?
I never tried it, I only made headshots to be a one hit kill for Barney, scientists, and hgrunts/assassins. so I made the number of damage lower... try using a negative number.. may work.
Do you code!?