Who knew _______ was a killer? (Spoilers)

There was a very clever yet subtle giveaway near the start of the movie that gave Chae Su-Yeon away. When Detective Cho asks her to look at the killer's movements (placing the garbage bag in the elevator), there is a quick downward shift in Chae Su-Yeon's eyes. Very very clever, and I must say great acting by Eun-ha Shim. Although I picked up on it, the film does terrifically well in getting you're mind onto other clues. For instance, how the killer always avoided confrontation, I'm talking about face-to-face, or fighting like "a man". The killer was portrayed as sneaky, and always using tools to tilt the outcome of confrontation to their favour. I'm talking about using the car to bang around Detective Cho, using a mini forklift(?) and knife to kill Detective Oh. It was only after I decided that the killer must be female did I out one and one together (Chae Su-Yeon and the killer's profile if you like). It's the little things that I enjoyed about this film, very well done.

Thoughts?