She felt herself swallowed up in the scorn of these virtuous creatures, who had first sacrificed, then rejected her as a thing useless and unclean.
The virtuous creatures, who represent the six rich people as well as Cornudet and the two nuns, think of "Dumpling" or Boule de Suif herself as someone of low status, someone shameful, dirty, not deserving their attention. However, when they are faced with difficulty, they readily use Boule de Suif to continue with their trip. Once they are safe again, they return to treating her with disgust, as if she wasn't the one they were asking for help hours ago.
At first, ill-suppressed wrath shook her whole person, and she opened her lips to shriek the truth at them, to overwhelm them with a volley of insults; but she could not utter a word, so choked was she with indignation.
Boule de Suif sacrificed herself only to help her fellow travelers. They gladly persuaded her that what she did was necessary for the greater good. Even when they had nothing to eat, Boule de Suif gave them from her own food. Yet, after they didn't need her anymore, they refused to talk to her; they didn't give her even food like she did when they were in difficulties.
What a mercy I am not sitting beside that creature!
With these words Madame Loiseau describes the contempt she feels for Boule de Suif, even that she required her help hours ago. She thinks that Boule de Suif is a very low status person who doesn't deserve her attention and who deserves being ridiculed.
I tell you all, you have done an infamous thing!
Cornudet shouts to his fellow travelers about what they did to Boule de Suif. While they are celebrating their "victory," he reminds them that what they did was shameful and not worthy to be celebrated.
