His mental illness has never been stated, but considering that he’s sociopathic, narcissistic, and cruel, and shows many symptoms of ASPD, it can be assumed that he may have ASPD.
for instance, What is wrong with Sangwoo?
Sangwoo is very much a sadistic, manipulative killer. He kills for sport and often snags his victims by inviting them home. Similar to Yoon Bum, Sangwoo also suffered from a life of abuse when he was younger, and now wishes to play the hunter instead of the prey.
significantly, What mental illness did Sangwoo’s mom have?
This would explain Sangwoo’s suspicion that there was someone else in the house at the end of season 2 (even though he actually wasn’t wrong) and it can ace include hallucinations: his mother. (Hence the title Borderline Personality Disorder: it is on the borderline between being a psychosis and neurosis.
also
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