Because they torment each other so much, Laurent and Thérèse seem to think that killing each other will help them move on from murdering Camille. The logic behind this thinking isn’t very good, since it’s unlikely that committing yet
another murder will help them forget about the first one. But they seem to think it’s impossible to go on living with the knowledge that somebody else in the world is aware of what really happened to Camille, which is most likely why they don’t just decide to leave each other—they would always fear that the other might turn them in. The novel thus shows how one heinous act can lead to another, as both Laurent and Thérèse justify committing another murder to cover up the first one.