Why did Dorian Gray kill Basil?

Basil Hallward had gone to Dorian's home to confront him about the rumors that plague his reputation, and they got into an argument. Eventually, Dorian offered Basil a look at his soul. He showed Basil the now-hideous portrait, and a horrified Hallward begged him to repent. Dorian claimed it was too late for penance and, in a fit of rage, kills Basil.

Explanation:

Basil refused to believe that his supposed friends, Henry and Dorian, were bad. Ultimately, he paid for his optimism and good faith with his life.