These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
At the end of the film, when Do-joon reveals the pins his mother forgot in the house where she killed the only witness who can confirm that Do-joon actually killed Ah-jung, and the mother finally uses them on herself.