Peter had a lot of regrets in his life. Fighting against the Sorting Hat's choice to put him in Hufflepuff and instead arguing to be in Gryffindor was his first. While that was why he had such great friends, it was also the cause of a lot of problems in his life.
The other major regret was accepting the Dark Lord's offer to join the Death Eaters. He had justifiably been quite scared for his life, but he should have stayed strong and rejected the offer. But he wasn't as brave as his friends, and he chose to do what was easy.
That was why he was on the way to the Potter's cottage in Godric's Hollow. He was the Secret Keeper of the house, and he had betrayed his close friends to save his own life. After revealing the secret to the Dark Lord, he couldn't stay still, so he was pacing around his house nervously.
He suddenly hissed in pain as his left arm felt as if it was on fire. He pulled his sleeve to see that the Dark Mark had suddenly faded, and rather than its usual pitch black, it was a dull grey. Peter did not know what had happened, but guessed something had happened at Godric's Hollow.
Peter quickly apparated to the house only to see the first floor in ruins. He rushed inside and saw James' dead body lying on the floor. Despite being the one to cause his death, he felt his eyes well up with tears in sadness. He pushed down that feeling and went up the stairs.
As he was going up, he heard a baby cry. Peter felt another rush of emotions, and he ran as quickly as he could. He saw Lily's body in front of the baby's crib, and a black cloak lying on the floor, along with the wand that he could recognise anywhere, the Dark Lord's.
Quickly snagging it, he hid it in his pocket, before taking his own wand and moving closer to the crib. Hearing Harry cry again, he took him in his arms and consoled him. He saw a bleeding scar on Harry's forehead so he cast what little healing charms that he knew on it, sighing with relief as that seemed to stop the bleeding temporarily.
He was startled as he heard a bike roar outside the house. Realising that it must be Sirius, he became anxious once again. Quickly concocting a plan, he prayed that he would be able to execute it without getting caught.
He heard Sirius shout and assumed that Sirius must have seen James' corpse. Peter carefully put Harry back in the crib, before running out to see Sirius.
"Padfoot! Voldemort killed them both!" Peter sobbed.
"You rat! I knew I shouldn't have trusted you, it's all my fault. I should have been the secret keeper myself. James and Lily wouldn't have been dead otherwise!" Sirius shouted in anger.
Peter tried his best to look confused. He was a good actor, he had to be, to ensure no one suspected him of playing pranks on the other students with the Marauders. Being a spy for the past few months helped a lot as well and the fear of getting caught was a great motivator too.
"What do you mean? I wasn't the Secret Keeper. I was for some time, but Lily wasn't sure if I could be trusted to keep quiet if I was tortured. She was right of course, I'm not as brave as any of you…"
"If it isn't you, it has to be Remus, but, but why would he do that? We are like brothers, he couldn't have betrayed us."
Peter noticed how Sirius was quick to believe he could have betrayed them, but didn't think Remus would do it. He couldn't blame Sirius, knowing the truth, but that didn't stop him from feeling resentful.
"It must have been, there is no other way this could have happened… Maybe his time spying on the werewolves could have led him to do this?" Peter said questioningly, trying to put thoughts into Sirius' mind and convince that it had to have been Remus.
Noticing that his statement had an effect on Sirius, he felt relieved. But he still wasn't out of the woods yet, he had to ensure that he never had to be questioned under Veritaserum. His mind racing, he thought of how to convince the Wizarding World that he wasn't the traitor. He smirked internally at the thought of pulling a prank of such magnitude on the entire world.
Sirius meanwhile was itching to just track down Remus and kill him. But as he was about to, he heard a baby cry. He was startled as he thought that Harry had also died along with James and Lily. He rushed to the crib and held Harry in his arms.
"Don't worry pup, I'll take care of you now," he whispered to Harry, his own eyes moistening. Even knowing Harry was safe didn't satisfy his urge to get revenge and kill Remus though. Maybe it was the famed "Black Madness" or it was just the rage of his friend betraying them that made him want to hunt down Remus.
After being friends with Sirius for years, Peter knew what he was thinking. Knowing things could go terribly if Sirius and Remus ended up talking, he quickly spoke up, "Padfoot, there are still many followers of You-Know-Who who will do their best to kill Harry once they realise what happened to their lord. It will be for the best if you could take care of Harry, I don't think I am strong enough to stop any attempts", he ended his sentence mumbling.
"You know that's not true Wormy, you are a great wizard, you are great at Potions and Runes–"
"That just means I'm useless in combat, you know that Sirius."
"What am I supposed to do then, I have no idea on how to take care of children… I doubt that I will do a good job at this Pete, and I don't want to screw this up."
"You owe it to Lily and James, Siri, don't let revenge consume you. Who knows what Remus might do, he might even make everyone believe it is you that betrayed the Potters, you're the 'Secret Keeper' according to the Order after all", Peter said, seeing that his words were having an impact on Sirius.
"You're right, I owe it to James and Lily, I'll have to try my best to raise Harry." Sirius said, slowly.
"Well, where do we take him now, Harry needs a healer, I tried my best to heal him, but I think it's better to get him to a professional."
Peter was relieved as all things seemed to be going relatively well. Regretting his decision to ever become a Death Eater, he swore to himself that he will make every effort he could to make Harry the best wizard he could possibly be. It was the least he could do now, after what he had done to his friends.