First trigger warnings: there is violence (torture, child abuse references, bullying etc) and some quite graphic images.
I watched on Viki, where it is titled "The Killer Inside".
This drama had plenty of twists and turns, intrigue, deception, threat and surprises.
I did decide fairly quickly who I thought the culprit was, and turned out right.
However, this drama kept me engaged right the way through and I watched all episodes back-to-back.
The cast was pretty good and suited their roles. Some very unpleasant and messed up characters. A little over-acting at times from some, but not too often. Occasionally it felt a little stilted/awkward too and several scenes were less than credible (the court scenes were a little bizarre - no witnesses or indication of what exactly the trial was of), but overall I thought this a decent watch and needed to know the outcome.
I did also wonder how a character could return to more or less where their life left off, after having been incarcerated for an unknown length of time (clarity around that would have been good). Hey, it is fiction though!
Credit to Yamada Ryosuke and Kawaei Rina, who I thought were very engaging.