Hmmm, I have to disagree. The Cullens aren't doing anything good??? They're going against all of their natural instincts to atone for something that was out of their control. If this is just "not doing bad", then consider their reasons for this particular (and immensly difficult) lifestyle choice.As for the Grace that the Cullens earn for giving stuff up... I don't believe they do. Setting aside Carlisle*, they're not doing anything good, are they? They've lived for years in a selfish little corner, struggling over their bloodlust. But they're not doing GOOD. They're just resisting doing BAD, which is what all of us do every single day. And, actually, in resisting doing the really, really, really BAD they do things that, in anyone else, we would consider criminal. But for redemption, not doing bad isn't nearly enough. Because what is their existence doing for the world? Absolutely nothing. They're so wrapped up in themselves that they have no time or energy to do anything for anyone else... they don't speak to people or other vampire any more than they have to, they spend the majority of their time locked away in the woods... that, for me, redemption and Grace do not make.
*Carlisle, I think, has the most to apologize for. How can you believe in a greater Beyond and then forever trap four people in a world of death, evil, danger, and constant pain?
They're aren't staving off hunting humans to "redeem their souls". As mentioned in New Moon when Carlisle was talking to Bella about the "soul issue", none of the others seem to share this particular belief. They have no prospects of going into heaven (their idea) so they have nothing to prove to "God", no worries of further sin. So heaven isn't the reason why they go against nature.
I'm sure a big part of it is to achieve redemption in their own eyes, to stop themselves from considering themselves as "monsters", however, a part of it, no matter how small, must be in empathy with the human race. To revolt against one's most strong and natural instincts, to spare others of harm or heartbreak IS good. Carlisle, especially who was so revolted with what he had become and battled alone with no one to guide or help him is the epitome of this kind of goodness.
You ask how Carlisle can believe in a greater beyond and then forever trap four people in a world of death, evil, danger and constant pain. Well, sure, Carlisle can believe in the THEORY that there is a heaven, but when faced with the imminent death of a patient, or Esme, his soulmate, isn't he allowed to act a little rashly. And in each of the cases, he had a good reason to change his family: Edward's mother specifically asked it of him, Esme is his soulmate, Rosalie, well, I think that was just his compassion and Emmett, to grant the wishes of his daughter. And besides all of that, he was lonely. He's only human after all.
Edit: And as for the redemption thing, Carlisle saves the lives of HUMANS everyday. His existence and consequent knowledge keeps them alive so they and their families can continue to live and love. And the reason they speak to humans and vampires no more than necessary is to survive. Humans are in danger around them , so they avoid them rather then form friendships (another exaple of good) and if other vampires were around, they wouldn't be the vegetarian ones, so, again, to keep humans safe. And besides, the Cullens to talk to and have vampire friends. There's a whole clan of them in Denali as proof.