short summary if you haven't watched the movie, koichi takakura, a former detective focusing on psychopaths & criminal psychology takes up an abandoned case from 6 years ago about 3 members of a family disappearing leaving behind their daughter, saki. at the same time koichi has moved into a new neighbourhood together with his wife, yasuko and they're attempting to get to familiarize with their new rather strange neighbours who also eventually have some involvement in the case. directed by kiyoshi kurosawa.
my interpretation on the end is that as seen in the scene where mio's mother is killed, mio is the one preparing the drugs and nishino trusts her with this because he knows that even if she did try to reach for help the story is unbelieveable, mio knows this too. i'm assuming she was supposed to give her mother something that would stop her from fighting back completely but instead she lowered the dose or switched it out. she probably messed with the injection yasuko gave koichi as well because he wouldn't be able to shoot nishino if it was full dose, otherwise nishino wouldn't trust him with the gun.
if this isn't what happened and koichi actually was given the full dose i think that the food mio gave to yasuko and takakura could have reduced the effects of the drugs because otherwise it wouldn't be shown (credit to some redditor)
something that i really didn't understand or appreciate is how the connection between saki and nishino is very very vague and barely understandable, personally i like when everything somewhat makes sense at the end rather than leaving you with barely anything. here we just know that 'nishino' maybe is the person who killed saki's family and the mizuto family.
another thing i heavily dislike is how many characters are plain sick in the head and that makes everything extremely unrealistic. for example how both the older officer and nogami went into the houses of suspects without telling anyone and how that was just left behind afterwards without being mentioned again. while i do really like kurosawa's style of movies and how everything does make sense when you think about it but that just doesn't make sense.
i do think it's quite a good movie if you're just competent and creative enough to understand the ending and connect everything together there's just some parts that bother me. i really like how it starts getting more and more obvious who is guilty, kind of like cure although cure is just objectively better and an absolute masterpiece.