Something like this should work, I believe:

if(!player.isGrounded)
{
    //calculate the distance between our current height and the height we were in the last frame
    lastYTravelDistance = player.position.y - lastYPosition;

    //if the difference is negative, it means we're descending 
    fallHeight += lastYTravelDistance < 0 ? lastYTravelDistance : 0;
}
else
{
    //we check to see if we passed the allowed falling distance and kill the player if necessary
    if(fallHeight >= -deathHeight)
       KillPlayer();

    //reset fall height since we landed (doesn't matter if we're dead or alive)
    fallHeight = 0;
}

//cache our current Y position for comparison in the next frame
lastYPosition = player.position.y;