Peter’s betrayal was complicated. It wasn't about that blood supremacy stuff. Maybe it was him being mad at them, thinking they had it coming.
He never knew how he ended up being a part of The Marauders in the first place. He supposes it was mainly because James felt bad leaving him. They were friends because of their history, not wanting to throw those years down the drain. Peter dreading being alone also played a role, preferring to be a last choice than not being one at all.
Peter knew he never really fitted in like the others did. He knew that he wasn’t like them at all. He knew that Sirius would pretend to need extra time of study when Peter came around, that Remus would read for a bit longer, that James would fly more laps around the quidditch court than he needed. He knew all of this, that he wouldn’t—couldn't—ever really fit.
He couldn’t say it didn’t bother him, because it did. It did bother him how Sirius would pick on him more than on James or Remus. It did bother him how Remus got a bored look on his face when Peter talked for too long and how he didn’t get it with Sirius or James. It did bother him that James interrupted Peter more than he did Sirius or Remus. It did bother him how the three of them would run off and leave him behind. It did bother him how they would have inside jokes he didn’t understand. But he just tried to put it in a box, bury it and never open it or look at it or think about it or anything.
They should’ve drifted apart when Hogwarts was over. “Oh I had a friend named Peter in my school days, wonder what he’s doing now.” “I heard he got married.” "Good for him." He wouldn’t have been any help to The Dark Lord that way. But no, they became animagi and couldn’t leave Moony alone—James's words. He never even wanted to become one anyway, he thought it was dangerous, but he also thought that he would be left behind if he didn’t do it. He also didn’t want to join the order, he wanted to hide away and hope that he would be safe—as the cowards did, Sirius’s words— but again, he didn’t want to be left out. So he did what they said, for so many years, became a pushover.
He became jealous, of how it was so noticeable if James was gone (unlike Peter, who had to put a lot of effort into not being left behind), of how Sirius said what he thought as he thought it and everyone was so amused by it (unlike Peter who had to think it over and over again, hoping it would make them laugh and praying that nobody said anything at the same time he did), of how Remus was able to say no—quite a lot—and still be a part of the group (unlike Peter who was too scared to even try). He too tried to put this on a box and bury it deep down in a and never open it or look at it or think about it or anything.
But it got harder and harder and harder. So one day he just decided to think about the boxes, to look at them, to open them. He then began to hate James and Sirius and even Remus. He hated them for leaving him out, for forcing him to depend on them, for never making him feel like he was enough despite how hard he tried. He hated them so so so much. He hated Sirius most of all, he who had picked on him the most, planned stuff for just the three of them, interrupted him the most, pushed him around the most, took so much time of Remus who was the only one who seemed (even if it was just a bit) to like his company.
So it was easy to say yes when The Dark Lord asked. They’d pay, for all of it. That’s all he wanted, to make them pay. To make them regret it. To become someone who they respected, even feared if it wasn't too much to ask of life. Being with the Dark Lord ended up being the same, he was pushed around and too scared to say anything. Agreeing to stuff he didn't want to do. Stuff he thought was too much for him. Stuff that woke him up in the middle of the night. Stuff that he regretted so much. Like what happened to James. Mind you, he truly never intended for what happened to happen.
He'd remember James’s face when he realized that he had betrayed them for the rest of his life and cry himself to sleep, because that wasn’t the way it was going to go. James was supposed to be Imperioed and thrown aside for them to kill Harry but not this. He was now staring at his friend’s dead body and Merlin- that was not the plan James, he- Merlin. Peter felt sick to his stomach. He had seen a dead body before, Marlene’s. But Marlene hadn’t stood up for him when he was being bothered. Marlene hadn’t come up to the new short chubby kid that had just moved in and nobody wanted to be around but had still called him his friend when he didn't have to. She hadn't tried—failing miserably, but tried—to make him feel part of something. He had tried to make him feel important.
What had he done? How was he so stupid to join Voldemort? Idiot, idiot, idiot. He was such an idiot, it wasn't James's fault and he didn't have to pay for it.
He left a while later, after he stopped puking. Everything later is kind of a blur. He was numb and didn't give a shit anymore. He remembers blaming Sirius for what happened to James. He remembered turning into a rat and going to see Remus, he saw when Remus was told and as he fell to his knees and then left. He doesn't remember what he did next. Was he in Diagon Alley? He couldn't put his finger on it. He remembers thinking about just falling off somewhere tall and ending it all but he was too much of a coward to do it. He remembers thinking about going somewhere else, to pretend to be someone else, somebody better.
He was going to go through with it, at least that was his plan until a ginger haired boy picked him up “Mum! Look, a rat! Can we keep it? Pleaseeee”