Disclaimer: JKR owns the rights. I just played with the story a bit.
Telling the Dark Lord everything hadn't really been an option in Peter's mind. Betraying his best friends, as he'd later come to think of it as, wasn't in the plans. It never was in his plans to hurt someone, to cause the deaths of the ones he loved so much. He never meant for Padfoot to go to Azkaban or for Remus to be left solitary and alone.
He never meant anything - except for the part where, on their graduation night, they whispered drunken promises that sounded like "best friends forever."
But each made their own choices; James chose to marry Lily and, together, they chose to have a baby. Harry was the happiest baby that Peter had ever met, though he hadn't been around many of them in his short life. Remus chose to go and work in the underground as a recruit for the Order. Sirius chose to be reckless and hapless and pull double time as an Auror and Order Member. He also found his time to be hopelessly devoted to his godson, Harry. No one chose to notice when Peter's excuses became weaker and weaker.
They let Peter slid further and further into Voldemort's grip. He - the Dark Lord, that is - had made Peter a very, very appealing promise: "I will not let any harm come to those you love." It was too appealing to Peter who knew that, as Marauders, you stuck with your brothers and you looked out for the pack, rather than the lone wolf (or rat, in this case).
Then everything changed. Severus Snape, someone that Peter had never grown accustomed to working with, delivered news to the Dark Lord that a prophecy had been made to Albus Dumbledore; it seems that old man was planning on opening the school the following year (there had been so many rumors swirling about that controversy that this alone shocked Peter). He had been interviewing the new candidate for the Divitation Profession. Rumor had it that she was related to some great Seer, although she had never made an accurate prophecy. But something about this one was so real and tangible; it was almost that the Dark Lord had expected this one to come about.
Peter, shaking and stuttering, was summoned into the Lord's lair. "You understand why I can't keep my promise to you, Pettigrew. Now do the best that you can to figure out what charms they'll be using to hide from me."
"Yes, my Lord," Peter replied with a very deep bow (although his frame shook as he released himself from it). He scurried off to dinner at the Potter's - he had been invited that night. Seems they wanted to have a chat with him.
Dinner was a lovely affair. Lily always outdid herself, but tonight was different. It seemed the effort was beyond tangible and the food was the best that Peter had ever tasted in his life. He had a feeling why he was here. And although he couldn't tell them that he was the rat that was leaking the news, but he could try to persuade them. With so many things going on in the world (and Peter had witnessed many of them first hand. In example, he had been on the guard that made sure that Regulus Black, Sirius' younger brother, had been "taken care of." Sirius still didn't know.), it should be easy to convince them to use someone safer.
"Peter," James said urgently while Lily poured their coffee with the lovely cobbler she had made steamed in front of them. Peter's eyes were locked on James, afraid of what he'd say next.
"Look, Pete, you're one of my best mates. Lily and Harry and me, we need to go into hiding. Will you help us?"
"How?" Buying time had always been a speciality of his.
"There's a charm. You'd be our secret keeper. Basically you're the only one who would know where to find us. You'd just have to keep the secret."
"I don't think that's a good idea," Peter responded softly. He hoped James wouldn't ask too many questions.
"What?! Nonsense! I trust you with our lives."
"Yeah, but... What about Sirius? Why can't he do this?"
"It's obvious, isn't it? He's too obvious. They'd go after him. You? You can go undetected more so than he could. Say yes, Peter, please." James' tone was pleading and Lily findally returned to the table. Her face was composed into an emotionless mask, but the corners of her eyes were pulled in worry that a twenty-one year old shouldn't know.
Peter sillily recalled something he had heard Lily's father whisper at her wedding to an uncle (or something). "Nineteen, too young to be married." When, a year later, Lily announced that she was pregnant, Peter imagined that the man said that twenty was far too young to be having children. It was far too young to be having children. By the time Lily was barely over thirty, Harry'd be off to school and start his training to enter the real world. That wasn't enough time. But it didn't matter; they wouldn't live to see their twenty-second birthdays, let alone their thirtieth birthdays. Harry would never see his second birthday.
Twenty-one is far too young to be this involved in a war; twenty-one is far too young to be dying for this nonsense. And Harry? He's barely begun to live his life. Why can't Voldemort chose a different family? Why's it got to be the Potters?
Lily took Peter's thoughtful silence as a rejection. "At least think about it," she whispered softly.
"I'll do it," he said at the same time and a smile broke out amongst the three of them. Lily and James' smiles were great, big beaming smiles that radiated and probably could have lit the whole room. Peter's smile didn't meet his troubled eyes.
After the spell was performed, the cruciatius curse was applied to him. It took ten minutes for him to break down and tell Lord Voldemort exactly where to find his best friends and their son. It took exactly ten minutes for Peter to ruin countless lives.
For years to come, Peter had dreams of James and Lily and Harry. The dream always ended with James softly saying in a disappointed tone that "I trusted you with our lives, Peter."
Sirius escaped from Azkaban and Peter knew why. The dog was coming to avenge the stag. Peter had made this rat's nest, but he sure as hell wasn't going to lie in it.
Life was full of choices, after all, and for the first time, Peter Pettigrew was choosing to fight back.
