<strike>Give the falling object the BULLET BEHAVIOUR. Then this will allow you to check the speed of it when it hits the player (or any other object). If it's > a certain speed (the object is falling), then kill the player, otherwise don't (the object is still).</strike>
Nimtrix is right, just set the object with the PLATFORM BEHAVIOUR, set the default controls to NO so you can't move it around and check for a collision and if the object IS FALLING. if both are TRUE, kill the player. The way I described it will do the job but is just more work than you need to do.