The revelation that Pip receives that Molly, with her strong wrists, has killed another woman over Magwitch is disturbing to Pip; moreover, that this brutal, criminal woman is the mother of Estella and Magwitch the father greatly disturbs Pip. For, now Pip's illusion that Estella is a young lady of class and refinement is completely shattered.
This revelation furthers Dickens's theme of Appearance vs. Reality. Throughout the novel, Mr. Jaggers has warned Pip to "Take nothing on appearance," but Pip has been deluded by his desire to be a gentleman; he believes that if he acquires wealth and education he will become a better person. As a child, Pip perceives Miss Havisham as his "betters" simply because they live in a mansion and possess money.
Having learned that Estella is from lowlier parentage than he himself allows Pip to realize that it is what a person is within him/herself that is of true value. After all, the illiterate Joe, the blacksmith and friend and father to Pip, is much more worthy of Pip's respect than is "lady" Estella, who has never learned to care for anyone else.