"Decision to leave" is a film with a very complicated plot, and I am not sure I really understand every twist of it. There is an alibi that doesn't seem so waterproof after all when you consider the possibility of manipulating the cellphone of an old lady. There is a scene in which I was not sure if it portrayed (film)reality or just an hypothetical possibility the detective was thinking about.
The good news is I think that you don't have to understand 100% of the plot to grasp the essence of the movie. The essence is that the film is a mixture of crime and romance, a detective falling in love with his suspect.
Director Park Chan Wook has made violent films ("Oldboy", 2003) and sensual flms ("The handmaiden", 2016). "Decision to leave" is both, but in a more subdued manner than in the rest of his oeuvre. In an early scene the detective and his suspect have had their first interrogation and at the end of it they seem more accustomed to each other than a couple after 15 years of marriage.
Detective Hea Jun (Park Hae Il) does not sleep well and so the association with "Insomnia" (1997, Erik Skjoldbjærg & 2002, Christopher Nolan) is quickly made. An association that becomes even stronger due to the foggy weather in some parts of the movie. Maybe some Dutch viewers (as the writer of this review) have also thought of "The 4th man" (1983, Paul Verhoeven) about a woman who's husbands all mysteriously die. The difference is however that in "Decision to leave" the suspicion is there from the first moment on, while in "The 4th man" it only gradually arises.
"Decision to leave" is beautifully shot. I already mentioned the foggy weather, but also the interior scenes are sometimes beautifully framed. Apart from that there are some shots in which eyes are very prominent, indicating that this is not a movie about action but about see and be seen.