James Vane is a simple character—he's totally motivated by love, jealousy, and revenge. He adores his sister, Sibyl, and swears to kill anyone that hurts her:
"He is gone," murmured Sibyl sadly. "I wish you had seen him."
"I wish I had, for as sure as there is a God in heaven, if he ever does you any wrong, I shall kill him."
She looked at him in horror. He repeated his words. They cut the air like a dagger. (5.27-28)
He's kind of a super-masculine man in a world full of more ambiguous males; a sailor, traveler, and all around tough guy, he seems to represent a world of brawn over brains that Wilde's not terribly interested in.
That being said, he's not actually a bad guy—after all, he feels truly awful when he thinks he almost killed the wrong man. Really, he's just one of fate's punching bags—James isn't clever or handsome enough to make it in Wilde's world.