Yes, the current floor is what the player is standing on (it is a trace), so it can only be below the player. You then check if the floor (what the player is standing on) is an enemy, and if so, kill it. So the only way the player can kill an enemy is to land on them; also works if the enemy jumps into the player from below.
You can alternatively do like the others said and use a dot product. But a simpler way would be to say "if the player has collided with an enemy, and the player is higher than that enemy, kill the enemy; i.e. if (HitActor == Enemy && Player.Location.z > Enemy.Location.z) Enemy.Kill().