Psychopathy and Sociopathy aren't a thing any more, it's Anti-Personality Disorder.

I went to college for Clinical Psychology and Psychopathy and Sociopathy are from the old version of the DSM. The newest version of the DSM, the DSM V, would categorize his behavior as Anti-Social Personality Disorder, in which Psychopathy and Sociopathy are now converged. He has some empathy but not in a way that would be considered "functional." The way he acts throughout the chapters are astoundingly synonymous with the actions of someone with Anti-Social Personality Disorder which is really difficult to do as it's still a very misunderstood disorder. I would also keep in mind the places where he does feel "remorse", as they aren't necessarily because he feels sorry but there is something that could cause a negative impact on himself. For example, when Bum cut his wrists, he didn't feel bad because of the way he made Bum feel but because if Bum died, Sangwoo would be alone. Now that last part could be of speculation. There aren't many times that he has shown himself to be apologetic so that is an unfair point to fixate on. The biggest point is that there could be other things other than Anti-Personality Disorder but without his backstory it is difficult to tell. It's important to note that the way he acts is not justified by the fact that he could/does have a mental disorder but take note of the way he acts along with Yoonbum's actions as well. The way they behave is not consistent but the meticulous detail that Koogi puts in would indicate that this is on purpose. I highly suggest looking more into Anti-Personality Disorder and its relevancy to the misunderstood use psychopathy and sociopathy.