Connor's code is only executed if the player that's on top of the other player received any fall damage while landing on him. If so, the player who's down receives 0.5 times the damage the falling player did.
If you want to kill the player on the bottom no matter what, simply hook Ham_Touch and check if either of the touched entities is the ground entity (pev_groundentity) of the other one. If so, kill the player.