Final answer:
James Vane is deterred from killing Dorian Gray due to Dorian's youthful appearance, which casts doubt on his identity as the man involved with James's sister, resulting in James's uncertainty and Dorian's temporary reprieve.
Step-by-step explanation:
James Vane is prevented from killing Dorian due to a misunderstanding. In Oscar Wilde's 'The Picture of Dorian Gray,' James Vane desires vengeance for his sister, Sibyl's, death, for which he holds Dorian Gray responsible.
However, when James confronts Dorian, he is deceived by Dorian's youthful appearance, leading him to doubt whether Dorian is truly the same man who was involved with his sister many years ago.
Moreover, Dorian's assertion that he has not aged and the confirmation of his identity by a woman who claims Dorian has remained unchanged for over eighteen years further deters James.
This fortunate confusion of identities and the passage of time save Dorian’s life, at least temporarily, as James is unable to reconcile Dorian's youthful appearance with the timeline of his sister's demise.