I think Jiang Cheng's mother might be Wen Peng's mother as well. In the scene where teenage Wen Peng looked for…
Wen Peng said that man’s child meaning ML is not the biological son of Wen Peng’s mother. (If it true that is)Also the boy said he wanted to go to the Police University .
I personally think a selfish person suits this show more than a psycho person. As people are usually more selfish and take actions that benefit them rather than being a psycho. Psycho could be Gong Yoo’s character.