I also think she coerced Sangwoo into hiding the body. And the reason why I think she still messed around is because the box of contraceptives that was also found with the rat poisoning.
You guys didn't inherit that keen sense from Sherlock Holmes, didya?
You can learn or with time grow up to become a sociopath but being a psychopath is sth that you're from birth. That's the key difference. So Sangwoo is a sociopath coz of his mother and the abusive household he grew up in, he's not a psychopath.
Side note: I think Sangwoo's mom coerced him into romantically falling in love with her over the years. I think maybe his dad and she were fighting that day because his dad found out that she poisoned him. Sangwoo intervened and it was the finishing blow. After he died, I think she still cheated around with other guys so Sangwoo, being romantically in love with her, killed her. If psychopathy is inherited, he got it from her.