Anyone familiar with the Harry Potter canon knows that (SPOILER ALERT for those who haven't read the books or seen the films) Peter Pettigrew, aka Wormtail, betrayed James and Lily Potter by revealing their location to Voldemort. We all know that, and we all hate him for it. What a douche.
But what we don't know is why, exactly, he chose this path. We're never told why Peter sold out his friends, leaving James dead, Sirius incarcerated for a crime he didn't commit, and the most vulnerable one, Remus, alone, left to pick up the pieces. (I have a lot of feels, okay?)
Recently my parents and I rewatched the films, and every time Pettigrew was onscreen I just felt so, so sad and would think to myself, what happened to you? How did you become this? I couldn't get my head around it.
But now I think I can.
It came to me the other day in a very easy and boring Biology lesson as I contemplated a world in which Sirius and Remus were a confirmed thing (long story). I remember thinking that, with James and Lily also a couple, Peter would be left as the awkward fifth wheel, and I felt pretty bad for the guy. Then it hit me- that's why he did what he did.
Allow me to explain. It's canon that Peter basically idolised James, and my personal headcanon that it's only through James that he became friends with Sirius and Remus. The whole MWPP dynamic appears to be James and Sirius, ringleaders of insanity, Remus, the more or less responsible one who stops James and Sirius accidentally killing someone, and then Peter.
Peter Pettigrew is the least intelligent, least talented, and least charismatic of the group. He is, and has always been, the least good Marauder. Seriously, ask anyone who their favourite Marauder is and I guarantee that no-one will say Peter, because no-one likes Peter. (Granted, that might have more to do with the whole selling-out-his-friends thing, but I think we can all agree that Wormtail was always crap.) But in spite of his chronic crapness and downright terrible Animagus form (more on this later), he has friends who have his back, and whom he looks up to.
With James and Lily a couple, however, James would obviously have interests outside of the Marauders, and, as a result, outside of Peter. The great and powerful MWPP alliance would still exist, of course, but in a slightly different form. And, post-Hogwarts, it goes without saying that, even with the Order of the Phoenix existing, there'd be some drifting apart. James would be closer to Lily, and Sirius and Remus would probably be closer to each other than to Peter, because Peter isn't all that great and the weak link is always the first to be cut. Harsh but true. It is established in canon that Sirius and James are BFFs and that Lily and Remus are also good friends. But where is Peter?
There's an episode of The Big Bang Theory where a devastated Amy uses a human brain (plus tumour) to explain what she believes is her place in her social group, and I'm going to apply this metaphor to the Marauders and Lily to explain where Peter fits.
Firstly, we have the creative, impulsive right side of the brain. That would describe Sirius and James.
Then, there's the logical, intelligent left side. That's Remus and Lily.
And Peter? Peter's the tumour. Enough said.
Perhaps Peter's place in the group is indicated canonically through the animal forms (and Patronuses, in the case of Lily) of the five.
We have James/Prongs, the stag, and Lily, the woman with the doe Patronus. We have Remus/Moony, the werewolf, and Sirius/Padfoot, the dog. And then Peter/Wormtail, the rat. Not only are James and Lily clearly marked out as a couple (being the male and female of the same species), but Sirius and Remus are clearly a compliment to each other (since they are both canine). And then there's Peter - not only the odd one out, but the smallest and least-interesting. Sure, he has a useful function in allowing the Marauders access to the passageway under the Whomping Willow, but that's about it.
And now, hypothetically, let's consider for a moment the concept of Remus and Sirius as a couple - the situation, from Peter's perspective, is even worse! He's now, beyond a shadow of a doubt, the fifth wheel (rather than just a bit of one like before). I'm not saying that this was definitely the case, but it's certainly an interesting thought.
So to recap, we have Peter Pettigrew, sidelined by his idol and not quite sure of his place in the world outside of Hogwarts, of Gryffindor, and (it seems) of the Marauders. We can assume he's finding life after school hard to adjust to.
When you feel isolated, you become bitter or broken - and when you're bitter and broken, and you've spent your whole life in the shadow of others, you're open to manipulation. I think Voldy and the Death Eaters knew this. They identify Peter as the weakest and offer him a chance to help them. I repeat: they single out Peter, the one who, until now, has been almost constantly overlooked, and they make him feel like he's valuable. Even James didn't really do this - Peter was the Potters' Secret-Keeper, but only because Sirius reckoned no-one would think he would be. He was the second choice, chosen for an important job mainly because he wasn't good enough to be. That's not an honour! That sucks!
People tend to assume Peter only passed secrets to the Death Eaters because he was tortured or coerced, but I disagree. My theory is that Peter did what he did of his own free will. Let's consider the hypothesis in the previous paragraph. Voldemort and co show Peter attention, and Peter - lonely, broken, cast-off Peter - would do anything to keep it. Because that's what people do. Peter sold out the people he'd once cared about, but who no longer seemed to care so much about him, to someone who promised him the things he's been denied - power and recognition. We see this desire for attention clearly in Harry Potter and the Prisoner of Azkaban (PoA) and subsequent instalments of the series. He begs Ron not just to save him but to take him back, in rat form, as his pet. His role in Voldemort's organisation is 'lackey', and he never complains - he does everything asked of him willingly, even cutting off his own hand. Peter wants to please.
There is, however, potentially a darker undertone to his actions, in which everything I've said up until now holds true, but with one difference; just as Voldy came to Peter, Peter, jealous and bitter, goes to Voldemort. Betraying James and Lily is his way of enacting revenge for being constantly overlooked. In their final moments, they'll have no choice but to realise that Peter did this. They would have to remember him.
Voldemort knew Peter was weak, and Peter knew Voldemort could give him what he'd come to think of as justice. It's a symbiotic relationship.
It doesn't matter exactly what happened, though. This post never tried to establish that. What matters is that I think I understand why it happened. I think I understand why Peter transformed from the fourth Marauder, to the ultimate betrayer, to the sad, pathetic, grovelling creature - almost more rodent than human - we see in PoA and onwards.
This might not be your opinion (who knows? Maybe your version is closer to the truth!), but this is what makes sense in my mind.
[NOTE: Although I reference Wolfstar, I understand that it is not officially canon. The Wolfstar paragraph can be ignored if you do not ship it, since it is largely irrelevant to the theory]
[NOTE THE SECOND: Since I first posted this, I have learnt that Peter would have HAD to have given up the secret of his own free will. I'll take that as evidence]
[NOTE THE THIRD: I know in PoA Pettigrew says he gave up the secret out of fear and coercion, but that doesn't quite fit with the whole 'free will' stipulation of canon, and, given that Peter tries to shift the blame onto Sirius before that, we can assume that Peter Pettigrew is a dirty liar and there are alternative explanations]