As the certifiable villain of the novel, James epitomizes the worst kind of vampire. He embodies the evil that the Cullens have worked so hard not to become. James takes an immense amount of pleasure in hunting his victims. In fact, he goes after Bella because he knows that the Cullens will defend her, therefore giving him an exciting challenge. Edward, with his mind-reading skills, is able to get into James's head and understand his motivations. Here's how Edward describes him:
"If you didn't smell so appallingly luscious, [James] might not have bothered. But when I defended you…well, that made it a lot worse. He's not used to being thwarted, no matter how insignificant the object. He thinks of himself as a hunter and nothing else. His existence is consumed with tracking, and a challenge is all he asks of life. Suddenly we've presented him with a beautiful challenge – a large clan of strong fighters all bent on protecting the one vulnerable element. You wouldn't believe how euphoric he is now. It's his favorite game, and we've just made it his most exciting game ever." (19.79.4-11)
Even when James has Bella alone, he hopes that Edward will come after her. In fact, it seems that what he truly wants is to use Bella to lure in a different, more challenging prey – Edward.
We should also note that James is doubly a villain in Twilight, since we learn that he was the reason Alice was transformed into a vampire. An old vampire, who evidently cared a lot for Alice when she was a human, transformed her so she wouldn't become James's next victim. Angry that his plan to feast on Alice was thwarted, James killed this caring older vampire (with a lot of pleasure attached to that murder). In sum, James is nothing but evil.