What evidence from Pride and Prejudice shows that Jane Austen was an 18th century moralist?
Jane Austen is a moralist in the sense that she moralizes about ethical conduct and even etiquette. We see many examples of Austen's moralizing in Pride and Prejudice.
One example of moralizing about ethical conduct can be seen in Elizabeth's response to Darcy's letter. Austen describes Elizabeth as feeling "absolutely ashamed of herself" (Ch. 13, Vol. 2). She "prided herself on [her] discernment," meaning she valued her ability to see things, understand things, and judge things (Ch. 13, Vol. 2). Instead, she has now learned that she completely misjudged both Wickham and Darcy and was very deceived by Wickham. She allowed Wickham's lies to affect her opinion of Darcy. In short, Elizabeth realized that she was acting unethically in wrongfully and prejudicedly judging Darcy. She also realized that her vanity led to her misjudgement, which is another example of unethical behavior.
Examples Austen gives of moralizing about etiquette can be seen in the actions of the Bennet family, particularly the father's actions, mother's, and younger sisters'. To Austen, acting without proper etiquette is acting immorally. The father acted without moral etiquette when he failed to restrain the behavior of his wife and daughters, especially at the Netherfield ball. Mrs. Bennet frequently acted with immoral etiquette when she acted with impropriety. She frequently said socially inappropriate things, for instance she declared publicly that Bingley and Jane would soon be married, well before Bingley had proposed. The younger girls are guilty of immoral etiquette when they go off flirting with soldiers, endangering the family's reputation.