this is depiction of how jail time, no matter how short or long, will left a negative impact on pure souls. Eiher you wrongly accused or you are just accidentally kill a person without actually mean it, that experience will left a trauma and you might be never recover. This depiction is more about contrast to the police, they want the obvious suspect become a real suspect, because they think this is savest bet to level up. but they never know what they done to a suspect is evil and cruel.

either he is really a real culprit or innocent person, his view to the world during those experiience is total darkness and feels helpless.
this depiction is wonderfully convey in this little drama.

and this is after a very long time one of the best Kim Soo Hyun performances. which unfortunately not even given a nomination in baeksang. he should be doing this kind of performances more often.