Thérèse and Laurent start leading separate lives of debauchery: Thérèse has affairs with other men, while Laurent begins to drink and consort with other women.
They become increasingly suspicious of each other, each thinking that the other will inevitably confess to their murder.
The two realize that this miserable state of affairs can't go on forever, and the same idea occurs to them at the same moment: Laurent decides to kill Thérèse because he's afraid she will destroy him, and Thérèse decides to kill Laurent for the same reasons.
A match made in heaven, Thérèse and Laurent.
The next day, Laurent steals a flask of poison (prussic acid, to be exact) from the laboratory of one of his chemist friends.
And Thérèse sharpens a kitchen knife and hides it in the sideboard.