Just focusing on the story, this is chilling, well-made, and all too real. It's a sad commentary on humanity that this type of thing happens with depressing regularity. However, as one of the show's themes underscores: abused children may want revenge, but they seldom turn to life-taking.
The lack of continuity was disappointing. Characters disappear and reappear, as needed. We don't know what happened to some of them and for some, we don't know their backstories. Clearly, the focus was meant to be on the main plot. Rather chilling to think secondary characters were used only to drive it - rather like the main character confessed to doing, using and discarding people to drive his revenge plot.
Production values were a bit off. Scars appear on one side of the face in one scene and the other side, the next. The first murder happens in January - a notoriously cold time in Korea, yet there's no snow on the ground and everyone is wearing light jackets or open coats. The story gives no sense of pacing; we don't know how much time passes between scenes. However, scars disappearing from one frame to the next suggests quite a while passed.
This doesn't deliver a message, like Mouse did. It's not thought-provoking, as that show was. Still, if you don't look too deep at it, it's a decent watch.