@Mentally ill uhh as a rereader, it's a bit hard to explain........
Spoiler!
Honestly you can't blame sangwoo for being crazy, the blame is on his psychotic crazy mother who tried to kill him if I remember well. But he is stills red flag. Also from what I remember he does treat yoon b** a very very tiny bit better, but I think he goes back to treating him badly, then goes back to treating him goodl