
You can tell Kang Moo is a sociopath because he doesn't close his eyes when he kisses. That is sociopath behavior.

I was thinking more along the lines of aggressive, unblinking staring. But, if you are in a relationship with someone and you both enjoy leaving your eyes open, who am I to judge. lol
More seriously, I think it is interesting within the context of the story. Kang Moo having his eyes open and watching intently while Kirin keeps his eyes closed I think reflects their characters really well.

Joking~ I was joking~ They're fake people and my comment is not to be taken seriously~
For the record, yes, psychopaths and sociopaths are terms often understood to have somewhat different meanings, but are not official diagnoses in the DSM5 and are therefore not well-defined. However, they are both often associated with the actual diagnosis of antisocial personality disorder and are therefore often described in a very similar way, if not interchangeably.
(Note that this is coming from widely accepted assertions in US psychiatric research, as far as I understand it.)

If you look at WebMD, it says what I said. Neither is an official diagnosis in the DSM5. They are understood to have slightly different meanings (under the header of ASPD), but are often used interchangeably in pop culture because *they are not an official diagnosis.* (None of which matters here, but seemed like some fun information to share.)
Also, I was joking. It was a joke. I don't know if you missed where I said that. Not diagnosing. Not striving for accuracy. Not doing a canon RP in the comments. Joking.

I understand. For the purposes of this story and any serious discussion of characters, I go with the author established definitions. I don't think anything else matters as much as what the author is establishing to be true within the context of the story.
I also want to note that the author is not in the United States so they may be pulling from research and established definitions more local to them, rather than relying on DSM definitions that are established by the American Psychiatric Association. So, citing WebMD/an American psychologist doesn't necessarily help determine what is or is not "correct" for the author or help us to better understand their characters.
SO, to anyone who read my joke: Understand that it is a joke and has nothing to do with the author established definitions of psychopath or sociopath, which may or may not connect to definitions of the terms in the US. I am not challenging their characterization nor was I asserting the interchangeability or difference between the terms.
...But, seriously. If someone is staring you down hard when you are kissing, they're probably a sociopath. Or a psychopath.
...
(KIDDING! ...Or am I?)
I love reading the comments as people hate-read this webtoon. Everyone is so mad and I am here for it because I am mad too. lol