The guy at the end...

That guy really needed to die. It wouldn't matter if any other sh*t in the film lived as long as he didn't. I don't care if some people on this board would think it was ~pandering or a supposedly happy ending. I watched this to see action and for the revenge plot, not a gritty, realistic "life sucks" kind of film. They kinda threw realism out the window all throughout the movie anyway so I don't get why it would end that way.
I kinda wanted Taesu to stay alive, maybe develop an odd partnership with the little girl where he kills people for her (starting with the guy who beat up her sister). I guess that would drastically change her personality, but she was pretty young and I can see the things happening around her warping her mind a lot. Plus Taesu seemed to like killing people for some kind of approval.
And finally on the subject of Boksoon, after finishing the movie I felt like she wasn't even needed at all. Her and Nari's relationship was cute and all, but in the end I don't feel like she added much to the story aside from some classic Korean genre whiplash.