This scene reveals that, apparently, David has put considerable effort into planning his spree—it even has a designated end date, though thankfully Makani and her friends have prevented it from lasting that long. Despite all his planning, though, David’s plan makes little sense. It’s totally unclear what he thinks he'll accomplish by killing. The senselessness of David’s plan shows how significantly his inner demons warp his sense of reality. They’ve driven him to believe that violence is the only way to solve his problems—that he can evade his personal issues by lashing out at others. The knife symbolizes David’s inability to be introspective.