You haven't read the manhwa properly obviously. Because if you did, in an earlier chapter when a man tries to intrude the apartment, and woojin beats him up and they go to the police, he asks Haegyung why he was shaking like a leaf since he wasn't afraid of things like that before. He even asks him if its because an ex did something to him which was one of the first signs the author gave us about how Haegyung changed as a person because of some traumatic event