Somewhat off-topic, but this thread brought to mind an article I read recently, though it was about China, not Japan: Why drivers in China intentionally kill the pedestrians they hit: China?s laws have encouraged the hit to kill phenomenon.

Article TL;DR: In China, it is less economically damaging for someone to KILL a pedestrian than to simply injure them, so people involved in accidents sometimes run over their victims on purpose after the initial hit, to make sure the person is dead.