Choices Quotes in Twilight
How we cite our quotes: (Chapter. Paragraph)
"It took everything I had not to jump up in the middle of that class full of children and–" [Edward] stopped abruptly, looking away. "When you walked past me, I could have ruined everything Carlisle has built for us, right then and there. If I hadn't been denying my thirst for the last, well, too many years, I wouldn't have been able to stop myself." (13.100)
Edward, like the other Cullens, has made a conscious choice not to prey on humans. It took every bit of willpower Edward had not to suck Bella's blood upon first meeting her. For him, it's a battle between his monstrous instincts and his more human conscience and intellect.
[Edward to Bella:] "But I resisted. I don't know how. I forced myself not to wait for you, not to follow you from the school. It was easier outside, when I couldn't smell you anymore, to think clearly, to make the right decision. I left the others near home – I was too ashamed to tell them how weak I was, they only knew something was very wrong – and then I went straight to Carlisle, at the hospital, to tell him I was leaving." (13.109)
Edward originally sees leaving the area as his only option if he's going to maintain his "vegetarian" diet. His longing for Bella's blood is overpowering, and after how hard he and the Cullens have worked to maintain their diet and blend into the human world, he doesn't want to destroy the life they've built.
"No, that's just Carlisle. He would never do that to someone who had another choice." The respect in [Edward's] voice was profound when he spoke of his father figure. "It is easier he says, though," he continued, "if the blood is weak." (14.22)
Unless a person is dying, Carlisle would never transform them into a vampire. He sees it as a last resort. Later, when Edward refuses to turn Bella into a vampire, we learn that he shares Carlisle's view on the matter.