Laurent is the yang to Camille's yin. He's described as having a "sanguine beauty" with "swelling, well-developed muscles" and "thick, firm flesh" (5.10). Yowza.
The keyword here is sanguine, meaning blood-red and/or having a healthy complexion and an energetic personality. As you know, Camille is always sick and always boring—so Laurent really is Camille's polar opposite.
Now we'd like to interrupt your regularly-scheduled programming here to give you a brief history lesson. Because Zola's study of human "temperaments" in this book is loosely based on the medieval theory of humours.
Way back in the day (around 400 B.C.), a Greek doctor named Hippocrates proposed that our health is determined by four fluids: blood, phlegm, yellow bile, and black bile. To stay happy and hale, these four humours have to be kept in the right balance. If one humour dominates the others, then one of four temperaments may result: sanguine, melancholic, lymphatic, or bilious.
Being sanguine is a result of having too much blood in your system. Which is total baloney, as far as modern science is concerned, but can you see where we're going with all of this?
As a writer interested in the sciences, Zola was aware of this theory of the four humours and adapted it in his "study of temperaments," Thérèse Raquin. So Laurent's humours are all out-of-whack, which results in him being sexually insatiable. Zola calls his interest in sex an uncontrollable "animal appetite."
Remember that Zola's goal in Thérèse Raquin is to study what happens when people with different temperaments come into a contact with each other. Thérèse's nerves and Laurent's blood are supposed to balance each other out.
But when Laurent is prevented from meeting Thérèse, his system goes haywire. So, obviously, he starts plotting to kill Camille. It's only rational… if you believe in all this humours bunk.
Take a look at this passage from Chapter 9, when Laurent first begins to consider murdering Camille:
[H]e was reviewing the notion of murder. The idea of death, uttered in desperation between two kisses, came back keenly, relentlessly. Driven by insomnia, aroused by the pungent scents that Thérèse had left behind, he devised traps, working out what could go wrong and enumerating all the benefits to be derived from becoming a murderer. (9.49)
At this point, Laurent and Thérèse have just had sex, after not seeing each other for several weeks. And Laurent realizes that he can't survive if he doesn't see Thérèse on a daily basis. It's this necessity that drives him to consider murder.
Laurent, then, is framed as being completely controlled by his needs. He is "driven by" insomnia and his lust for Thérèse, and so he simply must kill Camille. Zola wasn't kidding when he said he wanted to investigate the animalistic—and deterministic—elements of what it means to be human.
But does Zola ever contradict his own desires to present a perfectly objective character study in Laurent? As Laurent falls deeper into paranoia at the end of the book, the narrator continues to insist that he has "no soul."
But riddle us this, Shmoopers: can Laurent's mental breakdown be understood merely as an "organic disorder"? Or is he also suffering from a disease of his soul-parts?