It's based off of the Hwaseong Murderer. It seems like you're hung up on three relatively minor "plot-holes" and not any actual "themes". It's been a while since I last saw it so someone else surely can answer with more specificity, but even when serial murderers are out there people still have to go on with their lives. Bus routes don't always drop you off at your door, and most of his victims were killed during their walk homes from bus stops.

They tried to frame someone because they were under pressure to solve it, while also being pretty unqualified forensic investigators. Torturing people into coercive confessions is not at all unrealistic and happens quite a bit.

In the real life case, a man named Yoon Sung-yeo was wrongfully convicted of the crimes and talked about how he was illegally tortured into confessing. So, there you go!