I think anyone with the knowledge and resources could create a Parasite or Living Weapon, so it's probably been multiple people throughout the centuries, not just one single person. I think Alex Ahad answered a question like this on his last Q&A, I'm not sure though.
It's confirmed that Parasites can turn on their host, but I don't think Living Weapons are capable of doing that since they are useless without someone to wield them. The worst that they can do to a wielder is not work for them.
Parasites are definitely more dangerous though, the fact that they are as intelligent as humans suggests that.