Symbolism, Imagery, Allegory
Ghosts are everywhere in this book. Which is a little odd, given the author's attempt to get rid of everything that's not scientifically verifiable.
Remember when Laurent and Camille struggle in the boat, and Camille bites Laurent's neck? That bite—Camille's last act as a living human—leaves behind a large scar. After the murder, Laurent begins having gruesome hallucinations of Camille's drowned body.
So this scar becomes a constant physical reminder of Camille's death. And Laurent's frequent hallucinations surrounding his scar—how it turns bright red and feels like it's burning—seems to be a pretty obvious symbol for the guy's guilty conscience.
And that guilty conscience seems to quite literally be driving him mad.
Zola does try to describe Laurent's hallucinations as the symptoms of a nervous breakdown. But the question is whether he succeeds in convincing us readers of his perspective. Can we really ignore all of the symbolism we see in this scar stuff, and understand Laurent's crazy behavior as nothing more than the malfunctions of a screwed-up-person's body?
Not bloody likely.