I don't really mind a shift in focus to the killer, but how does that come about? As an anime, Erased is pretty non-committal about characterizing the teacher. Behaviorally he's depicted as fairly normal, so the shift to a character spouting absurd ideals who throws his life away over what seems like nothing is pretty jarring.
And the scene of betrayal just feels so flat. For all this build up, the killer identifies themselves to the world just to buckle Satoru into the passenger seat and let him drown in a car. There's not really a physicality to things, or a back and forth, or anything that seems like it'd sate this new side of the teacher revealed inside the car ride.
On the other end of things, Satoru doesn't feel particularly sincere with how he operates at this point. His emotions feel heavily concealed, and there's not much background or insight offered into the character. As the story heads into its final stages there's a greater importance placed on who Satoru was and who he is going to become, but I don't really feel like the show has offered adequate perspective into Satoru's life. In a world of absent father figures, what was his dad like? What kind of manga artist was Satoru and what kind of views did he hold? Time after time it just seems like his desire to be a superhero is being pushed, but it's hard to even tell how selfish that wish is.