This drama is one of the best Korean dramas in terms of creativity, plot, and execution. The essential idea of the protagonist believing himself to be the psychopath serial killer, and living the role and becoming fearless when confronted with any situation is really perfect. It was really enjoyable to watch. Of course, the interactions he had with the actual psychopath were the best part of the whole show. It really made me think this is one of the best remedies for people struggling in their social life due to low self-esteem. It's like a self-fulfilling prophecy, or a fake it till you make it mentality but he is not aware that it's fake.

The actors all did a great job in performing their roles, I would say. Seo In Woo's actor choice Park Sung Hoon is really ideal in terms of general looks and vibes, and his acting was also really suitable.
Yoon Shi Yoon is also a really interesting and fitting choice for the character. His innocent and young features makes him much more comedic as a psychopath. He sometimes goes all out and crazy, and though it's exaggerated, it did not feel too much or irritating to watch. If someone else had done it though, it probably would be.
Jung In Sun in terms of acting was really great also.

While watching the show, I had these slight criticisms.
First of all, the officer Shim Bo Kyoung is far too smart. In almost every situation she is doubtful and so calculative, she theorizes and catches on the slightest things so perfectly. It is way too staged and planned. I understand her background with her father and all, but they made her this ideal intelligent and fearless officer, which is not realistic nor believable. Especially when everyone around her is dumb-efied; the higher up you go, the less intelligence they have. (Which is why she wanted to stay as a neighbourhood officer, I suppose)
Another problematic point is that, Dong Sik always tried to fit whatever information and idea he gains from those around him about himself into the mould of a psychopath, which is great, hilarious, and all, but it cannot always add up. He almost had no moment where he doubted his psychopathic self, or the diary and its origin. He cannot just blindly believe that this diary is his. Yes, at some point he started changing his direction and as if by choice ending his psychopathic activities, but he still didn't doubt their truth. Only until officer Shim questions it in jail, and though he denies it first, it soon all sank in and he so quickly changes his whole belief. They should have had him have multiple moments of doubt and questioning before that last bit for it to all directly make sense.