The conclusion of Thérèse Raquin is probably one of the most melodramatic, over-the-top endings that you'll ever read. The action escalates quickly, as Thérèse and Laurent plot to kill one another. And at the most climactic moment, just as Thérèse reaches for the knife and Laurent pours poison into Thérèse's glass, they catch each other in the middle of their murderous schemes.
Their overwhelming horror pushes them to commit a double suicide, rather than murder each other. And Mme Raquin watches triumphantly over them. She really wanted vengeance for her dead son Camille.
The biggest question that the ending of the novel raises is how we are supposed to interpret Thérèse and Laurent's suicide. Zola wants us to read their death as an inevitable consequence of Camille's murder: they're no longer able to deal with the pressure, so they ultimately succumb to it.
Zola also insists in his Preface that they don't commit suicide out of guilt, because morality isn't relevant to his scientific enquiry.
You're probably getting a little tired of hearing all our constant references to Zola's Preface and his naturalist goals and his scientific project. We understand. We're sick of it too. So let's try something completely different.
Imagine this: what if you hadn't read Zola's Preface? What if you stumbled across the very first edition of Thérèse Raquin from 1867 before Zola had written his infamous Preface. This would change your reading experience of Thérèse Raquin pretty drastically, wouldn't it?
If you read Thérèse Raquin without any authorial suggestions as to how to understand it, wouldn't you read Thérèse and Laurent's mental breakdown as a sign of their guilty consciences? Wouldn't you interpret the lovers' suicide as conveying a moralistic message that murderers ultimately never escape punishment?
If you're nodding your head here, then this is where we begin to question the efficacy of Zola's attempt to turn literature into science. In the end, perhaps Zola's goal to turn novel-writing into science is all well and good in theory, but it just doesn't quite work out in practice.