I'm guessing that when a target is killed, the curse is lifted, and MC gets his MP back.
I understand that unequipping a tag means it can no longer track if you're killing someone with a tag, but then I don't see how having a tag protects anyone from being murdered. It's all too simple to unequip your tag and murder another. How does anyone ever end up with a red tag when there is such a simple way of bypassing it?