a possible explanation for those who need one
after first viewing 'mother', I felt slightly disappointed. the film seemed to have a great amount of potential ruined by poor pacing and plot issues. upon reflection and a second viewing, I came to understand that the pacing and plot of the film are probably it's greatest strengths, as much of what occurs in joon-ho bong's masterwork goes largely unexplained. it seems as if a number of people I know personally, as well as many IMDb members on the 'mother' message boards, had a similar problem, and can't quite make sense of the film, so I'd like to provide my own explanation for a few of the more complex elements within the movie. I apologize if my explanation seems at all convoluted or appears to reiterate what has been said on other boards.
firstly, and most importantly, the ending. do-joon is not, I feel, at all aware of the murder which his mother committed to defend him. the reason for the look of shock and horror on his mother's face, as well as her prompt exit from the bus terminal, is that she had, up until this point, not been aware of the murder she committed either. at the beginning of the film we see her dance in a field wearing a purple florally-designed jacket, blue dress and dark shoes. it's no coincidence that these are the same clothes which she has worn when murdering the junk-collector, as we can infer that after she murdered him, she gave herself the fabled acupuncture which can make one forget ones pain (after setting fire to the house we saw her wake up in a woodland area appearing both confused and refreshed, suggesting that she gave herself the acupuncture) - in other words, she would no longer remember killing the witness of do-joon's crime. she again does this aboard the bus to forget her ordeal, in an ending that is undeniably referential to chan-wook park's seminal south-korean film 'oldboy'.
secondly, the junk-collector. this is a character who had had sex with do-joon's victim at least once before (hence his picture being in her phone). this is suggested in the film his preparations for another sexual encounter (laying down a mat and even preparing a bowl of rice, supposedly the an appropriate currency with which the girl was to be paid). we can assume that the junk-collector was waiting her for her to cross his path again. when the junk-collector recounts his story to the mother, it is only natural that he would omit this detail, as such information would only serve to incriminate him and make him a suspect.
thirdly, and perhaps slightly irrelevantly, some viewers have expressed discomfort over an implied sexual relationship between do-joon and his mother. I personally believe that this is non-existent. do-joon says that he has "slept with" his mother to jin-tae, to which jin-tae replies "idiot". here, do-joon is mistaking the act of sexual congress for that of sleeping in a literal sense, as he and his mother share a bed. the sharing of the bed is most likely included to highlight the strength of the maternal influence which do-joon's mother holds over him, and the unnatural bond they share as a result.
I hope this has helped people understand a truly fascinating movie.