Disclaimer: I am not the great and powerful J.K. Rowling. Not even the wizard of Oz can grant that wish.

Author's Note:
Just to be clear, I am not a fan of Peter. This story is simply my personal quest to find out why Peter went bad. I went around and around in my head, and eventually came to the conclusion that Peter started serving Voldemort because he wanted to be part of something that he thought he fit into. All his life, Peter was forced into molds that he didn't fit. Anyone can see that he wasn't a true Gryffindor, and he was never really Marauders material either. When he joined up with Voldemort, he was trying to see whether he could finally fit into some group that he wasn't forced into.
Anyway, I hope this story helps you understand Peter's reasoning a bit better. I feel like there is a lot to be learned from Peter's story if we just take the time to look at it.

Peter was tired of not fitting in.

He felt like he shouldn't be a Marauder. He had nothing to offer them, nothing to bring to the table.

James was liked by everyone. He was friendly, funny, and charismatic, and everyone listened to him. He could score goal after goal as Chaser, and was good-looking on top of that. Though he often rubbed Slytherins the wrong way, and his strange relationship with Lily was a bomb that exploded erratically, he tried to be kind and helpful and avoided hurting others.

Sirius was a handsome devil. He had never been in a long-term relationship, though it took all the Marauders' fingers to count the number of girls he'd snogged. Sirius wasn't universally liked in the way James was; instead he was somewhat of an idol to those who didn't know him well. To his friends, however, there was much more to him than his status as Hogwarts's Biggest Flirt: Sirius was surprisingly clever and fiercely protective of his friends. His big heart for those dear to him often drove him to making reckless decisions.

Remus was the Marauders' voice of reason. Though studious and quiet, he marked his spot as a magical mischief maker with his wit. Remus was insanely good with words and should never be challenged to a comeback-battle, for he won them every time against Sirius, who was deluded enough to keep taking him on. Remus was an invaluable tool when it came to pranks, for he provided good ideas, but more importantly, he made the plans work. He could find flaws unnoticed by everyone else, he could organize the prank executions, and he could keep the teachers' trust all the while.

And then there was Peter. What did Peter have that made him a Marauder? What skills, traits, talents, did Peter possess that justified his position with the kings of Hogwarts?

Luck.

Peter only had luck. He'd had one tiny piece of luck to find himself in the same boat across the Lake with James and Sirius in their first year at Hogwarts.

Now, Peter didn't think his luck endured anymore. He saw things going wrong around him all the time. Didn't they always get caught when he joined his friends in sneaking to kitchens after curfew? Wasn't losing the Marauder's Map to Filch entirely the result of his careless mistake? And he knew he always said the wrong things when he was supposed to distract others from noticing James under the Invisibility Cloak.

Maybe he should just say it. Maybe he should just stand up and tell them that he really didn't belong, and that if they wanted to be successful they should just leave him behind. But he knew what they would say in return.

Remus would tell him that he was their friend, their brother in all but blood, and they couldn't very well leave him alone.

Sirius would point out that Peter couldn't leave the Marauders when he knew all their deepest, darkest secrets.

James would say that if Peter wanted to go, the rest would go with him and they'd end up all together again.

Then everyone would laugh and forget about the conversation. But it wouldn't matter anyway, because they couldn't take concerns like those seriously, they thought it was all a joke. To them, it was unthinkable that the Marauders could ever be split up.

And frankly, that was what bothered Peter the most.

Well, you have finished your little trip into Peter's head. Let me know what you think! (Hint: review button!)