So, I don't know much about S2 but my thoughts are, is the collector an Akuma? Well here is my thinking, Hawkmoth Akumatizes people who are mad, salty etc; Hawkmoth then tries to use the Akuma to control them to take Ladybug and Chat Noir's Miraculous and threatens to take away the Akuma's power if they try to fight back. But what if the Akuma is also a Miraculous holder. So, Master Fu said he only made a mistake when he gave out a miraculous once this is not Hawkmoth he is talking about because sometime in the 1st season he says that the Butterfly miraculous was lost a long time ago (and found by Hawkmoth). That mistake he made was giving a miraculous to a power hungry man (or Woman) let's just name him Bob. So Bob is power hungry with a miraculous but he doesn't know what to do with it. Bob gets frustrated about something probably kills someone and then Hawkmoth Akumatizes him. Hawkmoth loses control of Bob and instead 'The collector' (Witch is definitely a name Hawkmoth would give someone) controls Hawkmoth being a much larger threat to Chat Noir and Marinette being both Akumatized and Collecting miraculous's with two already in the palm of his hand.
Thanks for reading!