So, Darcy’s mea culpa at the end of P&P is interesting to me for a few reasons.

He goes out of his way to emphasize that he’s not talking about some development of the last few years, but his character from childhood—8 to 28.

His representation of himself is decidedly incompatible with the testimonies we heard at Pemberley, especially the one from Mrs Reynolds.

His insistence on Elizabeth’s innocence in her misjudgments conflicts pretty sharply with Elizabeth’s own dramatic mea culpa after his letter and her general sense of responsibility for her errors.

He manages to simultaneously argue that his faults were created by his parents and he didn’t know better until Elizabeth’s reality check, and that his own mistakes were monumental and inexcusable.

With regard to #1 and #2, I think the most likely explanation is that both he and Mrs Reynolds are (unintentionally) exaggerating—his remorse and relief leads him to overstate his failings from childhood on, while her affection leads her to idealize him and Georgiana, and dismiss all criticisms.

#3 makes it clearer that he’s not an entirely reliable judge in this interchange. He won’t blame her because, IMO, he’s too in love and thrilled, but the narrative does not excuse her to nearly the same degree. It makes sense that he would feel that way, for sure—this isn’t a criticism of the writing—but I don’t think we’re meant to fully adopt his view.

#4 is where it gets really interesting, because that is a very, very delicate balance.

It’s important to him that Elizabeth knows he isn’t fundamentally assholish, but rather, that his mistakes arose from how he was brought up, and he adjusted course as soon as he understood them. (Veering into headcanon a bit, I think part of the purpose of this is assuring her that she can trust that his current behaviour is what she’ll find in marriage.) At the same time that he blames his parents, though, he also idealizes them (especially his father) and avoids criticizing them directly, instead focusing on his own faults and moral responsibility for them.

That’s what I find most compelling about his speech, tbh. It manages to balance an awareness of the largely external factors that made him the way he was and to avoid wholly dismissing his responsibility for his own behaviour. I think there’s often a tendency to treat flaws based on upbringing as entirely dismissable (because other people created them) or to go in for the psychological bootstraps of insisting people should just magically know better than what they’ve been taught and simply lift themselves up by this age or that. But this walks between those two things, so that the impression for most readers is that he has flaws and made mistakes that he’s answerable for, but also that being influenced by dubious parenting doesn’t make him (or you) a fundamentally bad person.

I think that balance is one of the main reasons his character and development work so well for me. Even early on, we can see signs of his underlying personality as well as his flaws, and that underlying personality is revealed as better and better as the second half of the novel progresses, but his flaws are treated as significant, things that he is morally obligated to at least partially overcome once he understands them, while a few of them persist enough to be a) funny and b) credible. It’s just ... his character is put together in such an interesting, satisfying way overall, but especially in terms of his faults.