Late one night, Dorian runs into Basil Hallward on the street. Basil is delighted to see him, as he has been searching for Dorian all night, wanting to say goodbye before leaving on a six month trip to Paris. Basil has several hours before his train leaves, and the two adjourn to Dorian's home. The painter tells Dorian that he has been worried because "the most dreadful things are being said against [him] in London." Dorian is annoyed, and tells his friend that he doesn't care for gossip, but makes no effort to defend himself. Disconcerted by his friend's apathy, Basil goes on to assure Dorian that, vicious and damning as many of the rumors are, he doesn't believe them because he trusts that Dorian is a good person, and that "sin is a thing that writes itself across a man's face. It cannot be concealed." Dorian looks as young and innocent as ever, and Basil believes his eyes.
Once the artist begins listing the names of people whom Dorian is said to have led astray, Dorian rebukes him, saying that he doesn't know what he's talking about, and warning him to mind his own business. He argues that no person is without sin or temptation, and that corruption is not a thing that can be taught. Dorian only feels responsible for showing people their true selves. During their discussion, Basil remarks that he feels as if he doesn't know Dorian at all, and that in order to know him "I should have to see your soul." This sends Dorian into an odd state of defensive paranoia. Laughing, he tells Basil that "You shall see [my soul] yourself tonight!" Basil is confused and frightened by Dorian's words. He wants his friend to deny the charges against him, and is unsure whether Dorian's refusal to do so amounts to an admission that they are, in fact, true. To answer all of Basil's doubts, Dorian invites the painter upstairs, to view his "diary".
They ascend the stairs in Dorian's house, and enter the attic. Dorian tells Basil to open the curtain if he wishes to see his soul. Basil, thinking his friend is mad, hesitates, and Dorian reveals the painting himself. The artist is horrified, and at first doesn't even recognize Dorian in "the hideous face on the canvas grinning at him." He refuses to believe that it's actually his own painting, thinking it to be some "foul parody," until he recognizes the frame, and finds his own signature at the bottom. Dorian observes Basil's horrified reaction with apathy, and reminds him of the wish he made years ago at the painter's studio, right after the portrait had been completed. Basil is overwhelmed by disgust, unsure of what to believe, and exclaims that Dorian must have been a devil all along, and that if this picture accurately reflects the man's soul, that he "must be worse even than those who talk against you fancy you to be!"
He urges Dorian to repent, to try and save his soul, at which point "an uncontrollable feeling of hatred for Basil Hallward came over him, as though it had been suggested to him by the image on the canvas, whispered into his ear by those grinning lips." In a frenzy, Dorian grabs a knife and plunges it into Basil's neck, stabbing him repeatedly, and then holds him down until he stops struggling and dies, a pool of blood spreading out across the table and weaving through the feet of his chair.
Dorian is surprised at the ease with which he performed the murder. He feels relieved by the thought that the man "who had painted the fatal portrait to which all his misery had been due, had gone out of his life." He leaves the attic and determines that he will be able to get away with his crime, since Basil was supposed to leave for Paris that night, and since no one knew of his visit. He will destroy Basil's bag and overcoat, but in order to get rid of the body, he must call on Alan Campbell.
Basil speaks at length about Dorian's alleged sins, but never actually states what these sins are, only saying that Dorian's "name was implicated in the most terrible confession I'd ever read." This propensity for only indirectly acknowledging the breaking of social taboos is an interesting tendency found in Victorian society, one shared by the narrator of Dorian Gray. We have read that there are rumors of Dorian's misdeeds but unless we witness them first-hand, as we do the murder, we never learn what they actually are. Like Basil, we can only assume the worst, based on the hideousness of the portrait.
That Wilde chooses to portray Dorian's transgressions in such a manner is worth noting. The narrator is clearly omniscient: he certainly appears capable of informing us about what, exactly, Dorian has done to spark so much gossip and disdain, but by only hinting at the nature of Dorian's transgressions, Wilde establishes a palpable sense of their illicitness, leading the reader to look for clues while also reinforcing the sense of Dorian's degradation.
Basil's condemnation of Dorian's sins, and his fervent desire for Dorian to repent, indicate a religiosity in the artist that was absent in our last encounter with him. Basil has acquired a sharply refined ethical sensibility. This may explain the decline in his artistic output, since Wilde states in the preface that "An ethical sympathy in an artist is an unpardonable mannerism of style." This "unpardonable mannerism" is partially responsible for Dorian's murderous rage, as it offends his artistic sensibility, which is the only claim to purity that he now feels justified in clinging to. We are, however, told that the murder is prompted most directly by the portrait itself: "an uncontrollable feeling of hatred...came over him, as though it had been suggested to him by the image on the canvas." The image confronts Dorian with his shameful life, and Dorian blames Basil, the painter, for the pain that he feels.
When the artist confronts Dorian, it is too much for him to bear, and he is driven to murder by "the mad passions of a hunted animal." Ever since he first encountered Lord Henry, Dorian has made a point of surrendering to his passions. Now, even the urge towards murderous violence cannot be checked. Try as he might in later chapters, he is never able to write off this crime as simply another new and exciting "artistic" experience, as he was able to do with Sibyl's death.
Violent images involving knives are found in several instances throughout the novel: in addition to Basil's murder, they are found when Basil threatens to destroy the portrait in chapter 2, and when Dorian reflects that he has killed Sibyl as if he had "cut her little throat with a knife" in chapter 8.