How does this quote exemplify several themes of The Great Gatsby?
"They were careless people... they smashed up things and creatures and then treated 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..."
With this quote Nick sums up his attitude towards the Buchanans and the irresponsible way in which they use their wealth and status to satisfy their own selfish desires and whims. The reference to "smashing up" both "creatures" as well as objects of course refers to the way that they were responsible, at least in part, for the "smashing up" of Jay Gatsby before his death. It was only an accident that led to Gatsby being killed instead of Tom Buchanan, let us remember. Tom was responsible for the literal smashing up of Myrtle Wilson because of his desire to have a mistress, and at the same time Daisy is responsible for the more emotional and psychological smashing up of Gatsby through the way that she treats him and plays him, before finally leaving him for Tom.
This of course echoes the central theme of this novel which is the irresponsible behaviour of the wealthy and rich. They, Nick declares, are able to do whatever they want then hid behind their wealth and status whilst others have to deal with the consequences. This is of course forcefully shown through the way in which Nick is left to deal with Gatsby's death alone.