I think this should be added, just as a rare way to PK people and for fun. You shouldn't be able to gain loot on any monsters or NMs you kill, and nobody should get loot from killing you either, except maybe some crappy stuff which is worth less than the amount of effort it takes to kill you.

It would just be fun fluff content. A raging, intelligent, sentient NM travelling through Eorzea (probably need to be on a time limit) and everyone who encountered you would obviously try to stop you from wreaking anymore havoc on innocent players.

People worrying about potential abuse are misunderstanding the concept of the system. It is obviously supposed to be abused! Go crazy with that thing, imo.