The picture of Dorian Gray
In this passage, despite a pleasant-looking setting, the character's mood is quite bad. He's actually thinking about his mistakes and his sins, and now he wishes he could change what he has done and redeem himself. The girl he was in love with killed herself because of his behavior; he remembers he confessed her what a horrible person he was, but she didn't believe him, since according to her wicked people were old and ugly, while he was a young, good looking man.
He doesn't blame himself for killing Basil who painted the picture, and he doesn't even feel responsible for Alan's suicide. He blames his beauty though, since it ruined him. The only good thing he has done was changing is mind and repenting. Perhaps a redemption would have made the picture look stunning again, so he decides to check it out. When he finds out the picture is still horrible he reacts desperately and tries to identify the causes which brought him to this wicked life; then, he stabs it and dies. Suddenly, the picture looks as perfect as it was when Basil painted it, and Dorian looks loathsome, exactly like the man who was in the picture before he stabbed it. His servants were able to identify him just thanks to the rings he was wearing. According to Wilde's logic, the character needs to be punished for his sins, since he was conscious when he was acting so wickedly; also, he didn't do anything to change his life, so the picture doesn't change as well. At first, the loathsome man in the picture looked extraneous to Dorian, but as soon as he realizes what he has done, he understands the picture is actually the reflection of his soul.
Dorian kills himself in the picture just to eliminate all the evidences which reminded him his wicked life, hoping this will also make him redeem from his sins. The portrait symbolizes the physical aspect of Dorian's soul. The conclusion of the book in consistent with Wilde's theories of art and life as stabbing the picture represents the murder of art, and, as a consequence, of Dorian's soul.