[Edward: "You have saved me," he said quietly.
[Bella:] "I can't always be Lois Lane," I insisted. "I want to be Superman, too." "You don't' know what you're asking." His voice was soft; he stared intently at the edge of the pillowcase.
"I think I do."
"Bella, you don't know. I've had almost ninety years to think about this, and I'm still not sure." (24.196-200)
Notice how Bella says that she wants to be "Superman, too." This reminds us of the time when Edward said to Bella, "What if I'm not a superhero? What if I'm the bad guy?" (5.109) Clearly, Bella has decided that Edward is a superhero (Superman, after all, is not a "bad guy"), and she wants to be a superhero as well. She doesn't think that vampires are inherently evil. Edward, however, isn't so sure. Unless Bella is on her deathbed and has no other options, Edward isn't keen on turning her into a vampire.