Any action or inaction in a position of power will always grant you enemies
Metzger just likes to watch all of the slaver recruits suffer by making them get a painful tatoo on their head.It also makes it so they can't change their mind, and in case they try to leave the slaver business, it wrecks their chances of having a "normal" life, since they would always been seen as a no-good slaver with that tatoo.