There are largely 2 main points addressed in the story.
1) What happens if someone creates a fast lane for all the public to deliver justice as they see fit?
2) What happened 8 years ago and how is it relevant to everyone involved?

Now whether what happened or happens is right or wrong, that's up to the viewers. They just tell you both stories. I'd say it's a well done show. From the practical case & investigation part, it's as good as any decent thriller out there. Nothing more though and nothing less either. As for the concept itself, it's a part vigilante / part revenge style story. But this wasn't an all-about-oppa show and with more people on different sides falling under the limelight, it's kinda hard to pick sides.
I have a strong bias for Park Hae Jin and one of the things many of his roles have in common is they give me a hard time seeing things as black and white. Everyone's performance and acting was good, Park Sung Woong, Kim Kwon, Im Ji Yeon (if she irritated me at times, I'd look at her outfits instead, they were good) and all the supporting actors as well, and so I was swayed this way and that and most importantly had a good interesting time watching this show. Certainly had its set of plot twists, big reveals, decoy plot lines and such. Beware of the last fake out is all I'll say.

Post that, the last scenes are setting up for a follow up story but that is none of our business until it's officially announced. This season reached its own conclusion. So can be watched for now.

Overall, 9/10 from me.