This is, obviously, the perfect time for Mr. Darcy to come see how she's doing—and then suddenly burst into a declaration of love.
She's stunned into silence, which he takes as a good sign—so he goes on about how he's tried to repress his interest in her because of her inferior position in life, which, dude. Way to propose.
The more he talks, the angrier Elizabeth gets.
She's planning to refuse him politely, but it's not very polite at all: she has no affectionate feelings for him, she announces, and she can't imagine saying yes to a man who is the reason that her sister is so unhappy.
Mr. Darcy tries to explain that he feels he did his friend (Mr. Bingley) a favor.
Elizabeth also points out that his behavior toward Wickham has been despicable.
Okay, says Mr. Darcy, maybe he shouldn't have led with how gross he finds her family—but he doesn't like to lie.
Elizabeth gets angrier and angrier and finally tells him, in short, that he is the last man in the world that she would marry. Oh, and also tells him that he's behaved in an "ungentlemanlike" manner (34.18), which is the nineteenth-century equivalent of an "F-you."