The one thing everyone seems to agree upon about Pettigrew is that he's a spineless coward.
I think Redboy makes a valid point there, somewhere.
Follow me... Pettigrew was good friends with James, Sirius, and Lupin - at one time - valued enough to be included in the invention of the very dubious Marauder's Map, considered "good enough" to be called a Marauder himself, helped through the process of becoming an animagus, and trusted enough (if only by Sirius) to be made Secret keeper. Also, Pettigrew was picked to be in Gryffindor - a thing he could not have done if he hadn't posessed some measure of courage - or loyalty.
Now, Voldemort is the biggest bully on the block. He got to Pettigrew - whether he tortured him physically, blackmailled him, or frankly scared the bloody bogies out of him - he forced Peter to do what he would rather not.
Up to the moment of Pettigrew's actual betrayal, he had a great deal of "good side" qualities. Pettigrew has a weakness. Voldemort found it. And Voldemort used it again and again to keep Pettigrew under his thumb.
Maybe one of two things will happen... Pettigrew will have an opportunity to pay back Harry where it won't "cost" him anything, but Harry will know he did it only because he owes him. The other thing that might happen is - because Pettigrew and Neville Longbottom have strange likenesses - Pettigrew will encounter Neville and see, like holding a mirror to his face, the full reality of his choices. Maybe Pettigrew will draw strength from Neville (also a timid, clumsy fellow) who had made far more honorable choices.

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