Also that Seo vs Shiina thing is the most direct Sociopath vs Psychopath example I have ever seen.
I am not very familiar with those pathologies. Can you explain the difference in this particular example?
So upfront, neither Psycho nor Sociopath are really used in modern mental illness definitions. The modern term is "Anti-Social Personality Disorder"
According to Bing the difference between them is:
Psychopaths have little or no conscience but are able to follow social conventions when it suits their needs. Sociopaths have a limited, albeit weak, ability to feel empathy and remorse.
Psychopathy is related to a physiological defect that results in the underdevelopment of the part of the brain responsible for impulse control and emotions. Sociopathy, on the other hand, is more likely the product of childhood trauma and physical, emotional abuse.
Sociopaths tend to act more impulsively and erratically compared to psychopaths. Sociopaths generally struggle to maintain a job or a family life, whereas psychopaths may be able to do so.
So based on this criteria, I'd say Shiina is a Psychopath and Seo is a Sociopath, we see in the manga that Shiina has no issue holding a friend group and she easily slots herself in to Satsuki's when she joins the team. She also doesn't go out of her way to hurt people outside of her bullying.
Seo meanwhile doesn't have a lot of close friends and the people she hung out with actively despised her. She is also much more likely to give in to her violent urges, as we see with her multiple attempts to murder people throughout the story. She wants to hurt people and she's willing to go out of her way to see it happen.
Though I re-iterate, neither of these terms is really used in modern medical science and what I've presented here is a much abbreviated version, the actual definitions likely vary a bit and my analysis may not hold up under more thorough scrutiny.