Note: This will contain massive spoilers for Persona 4
The most immediate failure of Persona 4 is that its central premise is bankrupted by the fact you already know who the killer is if you apply real world logic. Adachi going “oops” is poor writing. Anyone who looked at this plot logically using rational thought would figure out a Police Officer (especially a Detective) should not be going to a public place in broad daylight spewing confidential police information in front of high schoolers who are too dumb to figure it out because . . .the plot forces them to be stupid. Naoto explains a complete character profile when she joins, and it is very on the mark. Why didn’t she immediately realize it was Adachi? Because the plot needed to keep going for the Nanako kidnapping drama so Naoto doesn’t automatically place Adachi as the main suspect since he fits the psychological profile that she explained. They had to dumb down Naoto to keep the plot moving and she’s introduced as a genius detective.
The mystery and searching for the truth theme are absolutely ruined by Adachi because it’s completely obvious that it is Adachi. In fact, to understand how completely awful Persona 4 is at botching up its own plot, there’s a mystery figure in the foggy nights who expresses anger about people not dying after being thrown in. This scene is to build up the killer, the intensity of the narrative, and to give us an awesome reveal. But what happens? Turns out Adachi’s motivations don’t fit because he was just looking for a good time with deceiving Namatame and your group, Namatame thought he was saving people and not killing them so it couldn’t have been him, and Izanami wanted some vague “humanity will find its way” theme where she and Ameno-Sagiri basically leave it up to humanity. So, what does all of this build up and tension regarding the killer throughout the ENTIRETY of Persona 4 amount to? A Walking Plot Hole.
This is a murder mystery and the build-up itself is a plot hole, the fact Naoto makes a profile and yet doesn’t immediately conclude Adachi from her deductions is a plot hole, and essentially plot contrivance exists throughout the entire narrative to make the cast as stupid as possible to prolong the drama.
Finally: Why would Adachi, who is doing it for giggles out of boredom, even write a letter to Yu Narukami’s house telling him to stop saving people? What did that prolong? What was the purpose? How does it logically fit Adachi’s stated motivations?
Oh yes… PLOT HOLE!