For anyone confused by this chapter: Basically, Denji heard Kobeni crying out for help, and went to her rescue. He killed the ones who were berating her, then he realized where he was, and started asking for a Burger. Because of Kobeni tripping, another person berated her, so he was killed by Denji. Anyone who 'harms' Kobeni, or does something to her Denji doesn't like, is killed.
If he had been there to kill her, he would have done it as soon as she tripped the first time.