No expert on Korean law, but it seems like the mother made a mistake
SPOILERS
She killed the junk collector because he was about to call the police and tell his story. But if he had, wouldn't that have knocked the charge against her son down to involuntary manslaughter? He couldn't see the girl when he threw the rock, and he brought her to the roof in a misguided attempt to get help after his phone wouldn't work. How much jail time could he possibly get for that, when the police let him off with a fine for the incident with the professors, and that lawyer said he could reduce the punishment for the original charge to four years?