We have tons of dramas about good guys fighting for justice, because you see, rules can’t help and police is full of idiots. In reality those reasons why they couldn’t shut Shi wan in prison were valid, dying kid can’t make right judgment, mother who lost her child can’t think straight and one photo doesn’t prove anything. There was a high chance that Shi wan really ended up being innocent. Plus, it isn’t like all psychos end up being killers (ironically, they often become doctors).
Imagine, how cool it would be if by the end of the drama Soo-Jin achieved her goal and destroyed Shi-Wan’s life, but BAM, he is innocent and now she can’t change anything, all because her mother didn’t properly looked for her daughter. Her whole actions were dedicated by her guilt and sadness.
Main message would be that rules are there for reason and playing Batman in 21th century’s country might not always be a good idea.

I am thinking from a way different angle: guilty, but a bit older. College or something. I couldn´t feel real threat from a high school kid. a twenty-something would be way eerier in the sense of Yu Na´s case as well. Or seducing a high school girl and kidnapping her. and also more realistic. The son and the father should have been estranged / at total odds, the sons´ rivalry would have been better if he was older, but turned crazy after his parents covered up his multiple crimes. I would have given him some guilt as well. feeling guilty but doing things anyway, even when he tries to resist his urges.
turning out innocent would deflate the whole balloon.

what also would have added to the tension was if Yoon Seung-Ro had actual blood on his hands and that´s what turned the son mad.