The Picture of Dorian Gray

Dorian Gray (Hurd Hatfield) is a handsome, wealthy young man living in 19th century London. While generally intelligent, he is naive and easily manipulated. These faults lead to his spiral into sin and, ultimately, misery.
While posing for a painting by his friend Basil Hallward (Lowell Gilmore), Dorian meets Basil's friend Lord Henry Wotton (George Sanders). Wotton is cynical and witty, and tells Dorian that the only life worth living is one dedicated entirely to pleasure. After Wotton convinces Dorian that youth and beauty will bring him everything he desires, Dorian openly wishes that his portrait could age instead of him. He makes this statement in the presence of a certain Egyptian statue, which supposedly has the power to grant wishes.
Dorian visits a tavern, where he falls in love with a beautiful singer named Sibyl Vane (Angela Lansbury). He eventually enters a romance with her, despite the disapproval of Sibyl's brother James (Richard Fraser), and within weeks they are engaged. Though initially overjoyed, Dorian is again persuaded by Lord Henry to pursue a more hedonistic lifestyle. Dorian sends Sibyl a hurtful letter, breaking off their relationship and "compensating" her with a large sum of money.
The next morning, Lord Henry informs Dorian that a heartbroken Sibyl Vane had killed herself the night before. Dorian is at first shocked and guilt-ridden but then adopts Lord Henry's indifferent manner. He surprises Basil by going to the opera immediately after hearing of Sibyl's death. Returning home that night, Dorian notices a change in the portrait Basil had painted, which now hangs in his living room. The portrait looks harsher, and a shaken Dorian has it locked away in his old school room. He becomes even more dedicated to living a sinful and heartless life.
Years later, Dorian is nearing his fortieth birthday, but he looks the same as he did when he was twenty-two. The townspeople are awestruck at his unchanging appearance. Over eighteen years of pointless debauchery, the portrait remained locked away, with Dorian holding the only key. Dorian had grown more and more paranoid about the picture's being seen by others; he fires servants who he thought might suspect, and later a friend observes how often he changes them all. Over the years, the painting of the young Dorian had warped into that of a hideous, demon-like creature that reflects Dorian's sins. Basil eventually catches a glimpse of the portrait and attempts to talk Dorian into reforming his life. However, Dorian panics and murders his friend, leaving the body locked in the school room with the painting.
Dorian blackmails an old friend, Allen Campbell (Douglas Walton), into disposing of Basil's body secretly. He then enters into a romance with Basil's niece, Gladys (Donna Reed), who was a young child when the portrait was painted. Though Gladys had always loved Dorian (and is overjoyed when he proposes marriage), those who were once close to him begin to find him suspicious.
Dorian begins to realize the harm his life is doing to himself and to others. He is assaulted by James Vane, Sibyl's brother, who had sworn revenge for his sister's death. Dorian calmly tells James that he is too young to be the same man from eighteen years before. However, James soon learns the truth, but he is shot by accident during a hunting party at Dorian's estate while hiding in the bushes. Dorian knows he is guilty of yet another death and realizes he can still spare Gladys from the misfortune he would certainly cause her.
After leaving her a letter explaining himself, he returns to his old school room to face the painting. He notices a subtle improvement in the painting, due to his determining not to harm Gladys and he resolves to change his life. However he also resolves that the picture must be destroyed. He stabs his portrait in the heart to try to free himself from the spell, but then cries out as if he has been stabbed as well. His friends have realized what has been happening, and go to his home. In the schoolroom they discover that his portrait once again depicts Dorian as a young, innocent man; Dorian is dead on the floor, his appearance now reflecting all his sins.

Question: Why was Dorian assaulted by James
Answer: For revenge for his sister's death