There's nothing better than a man who's described as looking like a "sickly, spoiled child" (1.15), are we right, ladies? Not to mention being "small, puny and listless in manner, with a thin beard and his face covered in freckles" (1.15)? Yikes.
Too bad this is the fate that Camille must suffer. His "blood had been impoverished by illness" (2.16). If you read our "Character Analysis" of Laurent, you know that the author is interested in studying the human temperament in a way loosely based on humorism.
See, it's not Camille's fault that he's a kind of a terrible person. Zola emphasizes the effects of heredity and environment on his personality. We are told that Camille's inherently weak blood is worsened by his mother's excessive coddling, for example. In other words, Camille is the ultimate Momma's boy.
To make matters worse, Madame Raquin denies Camille an education by always keeping him at home. So his ignorance is said to become an "additional weakness" (2.5).
While Laurent has too much blood (hence the sanguine temperament), Camille has way too little blood, which means that he has a lymphatic temperament. Translation: Camille is both physically sickly and relationally uncool.
The main conflict of the novel results from the clash between Camille's weak disposition and Thérèse's fiery personality. Thérèse simply must find a way to balance her excessive nerves. So she starts up an affair with Laurent.
When Thérèse and Laurent are prevented from keeping their 'organisms' stabilized through this balance of nerves and blood, they resort to murdering Camille.
Believe it or not, Camille's presence only becomes strong when he dies and starts to haunt his old wife and friend. Or at least, that's what they believe. The gruesome image of Camille's drowned and rotting body seems to extend to every corner of every room.
It's only by dying that dude can move from a marginal position in the book to occupy center stage.
It's all very scientific, isn't it? … Well, that's one interpretation. You could also say that all of Zola's science is bunk, and love and sex simply drive humans crazy.