Overlay not appearing

So I want to make a sword which will display an overlay when you attack an enemy. I set the living entity hit with tool procedure on the sword to set an NBT tag for the source entity(which I assume is the player) to true. Then I set the condition for the overlay to if the NBT tag for the target entity(which I assume is also the player as I couldn't use source entity) is true. But when I attack an enemy with the sword nothing happens. Am I just missing something really obvious here because I can't see why it wouldn't work. I'd rather not use global variables because, unless I'm mistaken, there's no way to make them player specific.