Well to be honest, I understand the positive and negative reviews, let me explain.

To start, tell yourself that this is a series that begins on murders made 20 years ago, then we discover that a killer is spreading terror in current day as that was the case before.
Except the plot twist, there is a technology to find out if children have the genes of psychopaths.
Two children, who have the genes, will be the main focus of the series, the mysterious events of the past keep us in suspense on the unfolding thoses of the present, but above all, to know who are these two children, have they become killers ? And if so, who are they ? Are they related to the current day murders ?

The series has an incredible cast of actors who played their roles to perfection, especially Lee Seung Gi who delivers an incredible performance in the mind of a psychopathic killer.
They succeeded perfectly in playing on tones and colors, the series was dark and disturbing.

But also, I believe that along with Beyond Evil, this is one of the best Korean drama thriller and psychological series to come out in 2021.
Beyond Evil focused more on the victims, we lived the events through unreliable narrators, always linked to murders or disappearances years earlier, and although the main characters were morally gray and complex, we were fully in the perspective of the victims and how to rebuild or live with that.
Here, Mouse on the contrary has infiltrated the mind of a psychopath by playing on "And if he can feel emotions, regret ?", we also have here unreliable narrators, and the characters are intense.

I think overall, the show has a pretty good way of scripting how to portray the mind and reasoning of a psychopath even though there's a lot of fiction in it, like in any drama.

I was however a little disappointed by the plot, which I must say is not boring at all, from the beginning to the end, we are hypnotized.
What bothers me is the story that could have made an excellent psychological thriller even more intense and complex if they had extended the life of some of the characters, if they had forgotten some storylines, especially on the end which involves an organization in the upper class - a recurring plot in dramas theses days - and a conspiracy.
Also, I find that at times the answers were a little easy - while at other times it was not - and the real "bad guys" are quickly arrested and the evidence appears as if by magic - wrapped up in few episodes -, it was not the best end of a series that was so sensational.

I find, personally, that the plot could very well have focused around our main character and his identity, this struggle between good and evil which was abandoned far too soon in my opinion but which was most memorizing.
I would have liked to see more of Kwon Hwa Woon, his character was very intriguing and not used enough in my opinion.

But, Lee Seung Gi remains the highlight, he is fascinating in this ambiguous role, his acting and his character are perfect throughout his performance. I discover with pleasure this actor, who left me dubious at the beginning but who was exceptional.

Summary :
The characters were on top, nothing to complain about, the sets and the tone of the series too, on the other hand some points of the plot were weak - particularly at the end.
I recommend this series to fans of series with tension, action, suspense, psychology.
The overall rhythm is very addictive, I binge-watched the series.
I don't recommend to those who want a series in light tones and messages - it's quite heavy and mature - and romance, because there are little bits of it - and still it's not really romance.