Chapter 52: Much Ado About the Guest List
Thankfully, Elizabeth knows somebody among the wedding guests more than capable of dropping the deets on Mr. Darcy: her aunt. In as long as it takes a letter to travel out and back, Elizabeth learns that Darcy actually involved himself in the grimy business of flushing out Wickham in London. It was then he who actually paid off Wickham's debts and bought the army commission and pretty much made the whole marriage happen. That's why he was present.
This Mr. Darcy did out of a feeling of obligation; in his effort to be a consummate gentleman, he never publicly shamed Wickham for his past misdeeds—particularly the thing about trying to run away with Darcy's sister—and if he had, people might have been more wary around him, and this whole thing might never have happened. The clear moral of the story is, it is never, ever, ever appropriate, under any circumstance, to give anybody the benefit of the doubt.
But, whoops, that whole assistance was supposed to be a secret. Of course, now that Elizabeth knows this, she can't help but think, hee hee, but maybe he actually did it all for me. Then she reminds herself how dumb that sounds.
Wickham comes in one last time to resurrect some of the low-key flirting he did with Elizabeth in the past, asking about her visit to Darcy's estate which he'd heard rumor about. Elizabeth, however, isn't playing games anymore; or to be more precise, she's playing better games; when he asks if she made it out to the neighborhood where he was promised his parsonage—the one that would have suited him perfectly, he reiterates—she calls him out on his bullshit. She says, oh, yeah, I heard you didn't actually want to do the whole man-of-the-cloth thing so you gave it up willingly.
Properly shook at being confronted with the truth, Wickham just goes, "Uh, yeah, you don't remember me telling you that part? I'm pretty sure I mentioned that part", and then immediately peaces out for the rest of the story.