At the end of The Great Gatsby, Nick realizes that Tom and Daisy are very careless people.
Nick sees the hollowness of people like Tom, Daisy, and Jordan. While he might have been raised to refrain from judgment, Nick sees what Tom and Daisy do to the people who are cursed to care for them. Nick bears witness to how Daisy fails to stand up for Gatsby and the cruel way that Tom treats Myrtle. People like Tom, Daisy, and Jordan, who used to enjoy Gatsby's wealth when he was alive, are nowhere to be found when he dies. Nick realizes that people like Tom and Daisy are able to use their wealth as a buffer between themselves and the wonton destruction they wreak on others:
They were careless people, Tom and Daisy--they smashed up things and creatures and then retreated back to 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...
Nick understands the selfishness of Tom and Daisy. They show no responsibility. He recognizes how wealth and privilege have enabled them to sacrifice any human connection. This is striking to him as he comes to terms with the fraudulent nature of people like Tom and Daisy.