Before Peter Pettigrew became a Deatheater, no matter what anyone else might think, he truly had honourable intentions.
He wanted desperately to evade his ratty, scheming persona for once and what better way to do so than spying on the Dark Lord for the Order of the Phoenix?
Imagine it! Stupid, wimpy Wormtail - the only Marauder with the guts to put himself right in the line of fire! Just think how impressed Prongs, Padfoot and Moony would have been when they realised how much they'd underestimated their friend; when they realised that he had the nerve to do what none of them even dreamed of!
He didn't tell anyone at first because he didn't want to put them in danger; he knew they'd try and convince him not to do it, and why not? He wouldn't have believed of himself, either.
He really thought he'd be able to make a difference, to be the catalyst in the Dark Lord's down fall, to strike the fatal blow and let everyone live in peace once more. But he knew he'd have to make sacrifices so that You-Know-Who wouldn't suspect him. Every person he handed over, every life he maimed, every brownie point he earned; it was all for the Cause. His Cause, so that he wouldn't be discovered and then he'd be able to make it up by doing a thousand good things.
He became a good liar, but after a while he wasn't sure to whom he was lying. Voldemort? The Order? Dumbledore? His friends? Himself?
He did it all for the Cause, he told himself, as he went to the Dark Lord with the words that would give him enough trust and power to overthrow him once and for all.
"I can give you Lily and James Potter."