This line says it all:
they were careless people, Tom and Daisy—they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made. . . .
Tom and Daisy are the type of people Nick does not want to become. They are only concerned with money and appearances. Whenever they destroy something, they buy their way out of it. It does not matter to them who gets hurt in the process.
What they have smashed:
Tom is partially responsible for Myrtle's death and Wilson's suicide; he is directly responsible for Gatsby's murder. Tom leads Myrtle on and this is the path to her destruction.
Daisy is responsible for Myrtle's death and, in a way, for Gatsby's as well. Daisy leads Gatsby in in just the same way Tom does Myrtle.
Tom and Daisy take off near the end of the novel. They are traveling together.
The scene in which Nick describes Gatsby watching Tom and Daisy in their kitchen contains no dialogue between the two of them. We just receive the image of a silhouette of the two talking across the table holding hands. I find this image so important because to me it says that their relationship is now private, in between them. It also shows that they are in the midst of trying to repair some devastating blows to the fidelity of their marriage. They become distant and isolated from the society in which they used to participate, probably for their betterment.
They have smashed their cheating relationships.