Even if Peter made some bad decisions (some really bad decisions), he's still human. Has always been. And it makes me angry when people treat him like dirt in Marauder-era stories D:

Peter is a Gryffindor.

And he's in Gryffindor for a reason.

Let's go back to where it all began, shall we?

Imagine the Marauders at the beginning of their first year: small, young, ignorant of what the future will hold. James, likely, is the only one truly confident about himself. Sirius hates his family, but isn't quite independent enough to outright hope he gets into Gryffindor. He's just rebellious enough to realize that Slytherin is not where he wants to be.

Remus is lucky he's there at all, and he knows it; if Dumbledore hadn't become the Headmaster that year, he wouldn't be allowed to attend Hogwarts. He's been a werewolf for several years, since he was a small child, and knows exactly what the rest of the wizarding world thinks of his kind. He's, privately, very excited that he's at Hogwarts, but he's mostly terrified of the future and what it will hold for him.

And Peter, little Peter, is described as slightly overweight and mousy-looking. He's never been popular, been picked on in the past by his peers, magical and Muggle alike, and he isn't quite sure where he belongs (just like so many other eleven year olds). He doesn't know exactly what house he'll be sorted into, but he doesn't really care; he's more concerned with hoping that students here will like him better than the children back at home did, hoping that he will have a better life at Hogwarts than he has in the past.

They've all landed themselves into Gryffindor, and though Sirius, Remus, and Peter don't know quite what they're doing there (though the Sorting Hat does - the Hat always has its reasons), they decide to make the best of it. Sirius knows he's pissed off his family, knows there's going to be hell to pay... But he's Sirius, impulsive and loud and generally uncaring, so he strikes up a conversation with the boy he met on the train, James Potter. And since Lily is likely meeting Mary and her other roommates, sulking over the fact that Severus was Sorted into a different house, Remus and Peter are left to sit there quietly, observe their surroundings, try not to make anybody mad by breathing the wrong way.

But then James, always friendly and outgoing, notices his two new roommates by themselves and invites them over. Remus is so surprised that someone is talking to him that he agrees without question; Peter also agrees, because he doesn't want to make anyone angry by refusing such a generous request.

Peter was bullied as a child, was never liked by his peers. So he's formed an introspective personality, quiet and reserved and submissive, because he thinks that may make people like him more…or, at the very least, they'll find less about him to criticize. He's a natural follower, and James is a natural leader; soon, Sirius also steps into his personality, becoming James' second half. The two of them click like brothers, and Remus (who's also, naturally, more reserved, because of his condition and the backlash that may incur) and Peter are just kind of pulled along for the ride.

At the beginning of their friendship, surely, James and Sirius aren't as close to Remus and Peter as they eventually come to be. But to Sirius, he doesn't have a family, not really - Orion and Walburga Black are his parents only in name. He learns from James what real family is like, what real parents and siblings do, and he decides one day, in a very Sirius-like fashion, that James and Remus and Peter are going to be his brothers, because Regulus has been a piss-poor one so far. They've all become friends over the months, but to Sirius, they are all so much more than that…and the other three feel the same way.

James and Sirius are arrogant jerks, yes, and they bully students often during their time at Hogwarts. (Several of which, remember, are Slytherins who have hurt them in the past, harmed their classmates, and generally, in their eyes, deserve to be bullied. Even if that isn't the answer.)

But, clearly, this brotherhood between the Marauders has shone through the years. There are people they don't like; there are people they do like; and then there are their brothers, with whom they share anything and everything. There's a clear distinction between Severus and Peter, especially in James' and Sirius' eyes; Peter may not be as bright as them, may not be as handsome and talented at Quidditch, but that doesn't make him any less of a person, does it?

(The scene after the Defense OWL, you must remember, is biased. Remember that Harry is watching it, without true knowledge of how his father and his friends interact; he already hates Peter more than perhaps anyone in the world; he's going to subconsciously put him down, because in his mind, Peter-the-teenager and Peter-the-adult are the same person. But they're not; sixteen-year-old Peter is obviously much more carefree. He's not talented at Quidditch, has never had good reflexes, and one of his best friends enjoys playing with a nicked Snitch to improve his Chaser reflexes. So, of course, Peter enjoys watching him practice, if only to pick up some tips, or, at the very least, admire what he can never have. And Sirius' jabs about it? What kind of friends never joke around with each other - especially the four of them, who regularly enjoyed raising Hell in the school?)

He is clearly good friends with the other three; why else would he be let in on Remus' secret and their plans to become illegal Animagi for him? He didn't run screaming when he discovered that one of his best friends - brothers - was a Dark creature, and agreed to break the law - punishable by years in Azkaban - to help him. This isn't something the others could have forced him into; Dumbledore could have, easily, wiped his memories of Remus' condition. Instead, he accepts his friend for who he is, working endlessly to make it all just a little more bearable.

(And nobody can say he's a useless wizard, either, because he succeeds.)

During their schooling, of course, Voldemort is slowly gaining power throughout the country. This starts small, around their first year, and escalates to a climax by the time they're leaving Hogwarts. The four of them are scared; everyone is scared; of course they are. There's a mad sociopath planning to wipe out the majority of the country's population. They all join the Order - which is a militia group, full of wizards who must be ready to fight and die for the cause at a moment's notice - and begin to help with the war effort.

(They are eighteen years old.)

Peter is terrified, of Voldemort, of the war, of everything. This personality he's crafted for himself, this wall that keeps him safe from the psychological dangers surrounding him, can and will not help him here. But he's close with James and Sirius and Remus, and they're strong and brave and talented, so he naturally tries to stick with them as the war progresses. He's terrified that they will die, that Voldemort will win, and he's not quite as strong as his friends, mentally. So when he catches a whiff of the fact that Lily is pregnant, and that the baby may be in danger from Voldemort, he panics.

Peter starts feeding information to Voldemort about a year before the Potters are killed. This is shortly after Harry was born, likely soon after Voldemort decides he will be going after Harry and not Neville. Maybe he panics, trying to think of some way to save four of the most important people in his life; maybe he just can't take the stress of the war and the constant threat of death anymore.

(He's only twenty when Harry is born, remember.)

Whatever the reason, he turns to Voldemort, surely reluctant at first, because he has three brothers who would do anything to protect him. Of course, there are others as well: members of the Order, old classmates he is on good terms with, who he surely feels horrible about betraying...

Because this defensive personality that he's created for himself can't stand conflict, tries to make everyone as happy as possible. But in this situation, where he and everyone he cares about are under the constant threat of death…it's just not possible. That eats away at him, gnaws at his mind until he is teetering on the brink of insanity, because there's just no way he can guarantee that his friends will be safe.

But he does his best.

Voldemort has no problem torturing any of those around him, has no problem bending reluctant followers to his will. This doesn't just involve the Cruciatus Curse; he uses psychological warfare, perhaps promising Peter that if he feeds him information, just a little more, he'll let James and Lily live when he goes to kill Harry.

Peter's never been as strong as his friends, has never been good at dueling and figuring out strategies and keeping them alive, so he thinks this may be the best way to do it. If he sits around and does nothing, surely, James and Lily will die; they're his entire world; they and Sirius and Remus mean everything to him. So he agrees, feeds vital information to Voldemort…

(And he feels terrible that Caradoc goes missing, the McKinnons are killed, but they aren't his brothers, and he knows the alternative will be so much worse.)

And then when Voldemort kills his friends anyway, is vanquished that night, leaving Sirius with the blame and Peter with such horrible guilt that he can barely stand it…

Something deep inside him snaps.

And, the rest, of course, is history.