This man definitely hasn't lost all his memories he knows something

Spoiler





He lost his memories for sure... But i guess you can take the memory out of the murderer but you cant take the murderer out of the murderer themselves.

He definitely has.. But while his mind has forgotten, I think his body and instincts remember. You can tell in certain panels that even though his reactions are for the purpose of protecting Yiyeon now, which is good, he instinctively partially reverts back to his old self.