It's only later Peter realizes he doesn't belong in Gryffindor; he can't remember when exactly.
Maybe it's that prank they played when James got stuck behind the statue and Peter took the secret passage instead of helping him avoid detention. Maybe it's the time he notices the bruises and scars on Sirius's back and doesn't say anything, even though he's still got leftover healing potions from the last full moon. Maybe it's when he first thinks of all the ways he could kill his friends.
Maybe it's just because he's weak. He can't be anything special on his own, he knows that, he's known it for ages now. Always, he's tagged along, the supporting character, the sidekick, instead of the hero. First James, then Voldemort. Maybe it's because he can't help but pick the side that looks like it's going to win.
He
is
not
brave.
Never has been. And maybe the Sorting Hat knew, that first day at Hogwarts…
Not Hufflepuff, because he wasn't kind.
Not Ravenclaw, because he wasn't wise.
Not Slytherin, because he wasn't cunning.
So Gryffindor, because there was nothing (the only chance) left.
Peter used to think he could overcome whatever he was, but then he left Hogwarts and joined the Order with James and Lily and all the rest, and the McKinnons and the Prewetts were killed and he couldn't resist.
What was the use? He couldn't be like the others. He wasn't strong like them, he couldn't choose what was right over what was easy.
He joins Voldemort. Receives the Dark Mark. He is small, practically worthless, but he has a purpose. And then the Potters go into hiding, and Sirius is appointed their Secret-Keeper.
But then –
He switches, with Peter instead, and he can hardly contain his victory, ignoring the tiny corner of regret, because he is useful. He is necessary to Voldemort's plan. He is needed. He recalls a day, a conversation, when the Marauders speculated why their Animagi were what they were.
Sirius's was easy – loyal, companionable, protective, and sometimes fierce.
James's wasn't hard – bit of a show-off, proud, agile, and powerful.
But Peter – what was Peter? He was small, easily unnoticed, and sneaky.
He is a rat.
(He is a traitor)
He knows now.
And so Lily and James die, Harry doesn't, and he lives the next twelve years as a rat, with the Weasley family. He is the prudish one's pet for a while, Percy's, and then he is Ron's, and then Ron becomes Harry's friend.
Harry looks just like his father, except for his eyes. Harry has Lily's eyes.
Peter (Wormtail) is content for a while – his animal emotions are hardly comparable to his human ones – but when Sirius Black escapes from Azkaban, his fear is strong. He cannot eat. He cannot rest. What is he going to do?
Or, more accurately, what can he do?
He is almost caught, one night. Sirius (Padfoot) is there, in the same room with him, for the first time in twelve years. Sirius is after him. And he can recall Sirius's temper only too well.
So Peter fakes his death (again).
He is a rat. He hides in Hagrid's hut.
Then he is found (discovered). He knew it would happen eventually, though he was hoping to hide longer –
Then Padfoot finds him, he takes the boy down the passage underneath the Whomping Willow, like they used to do every full moon, only Wormtail was never as terrified then as he is now.
Sirius transforms, and he looks awful. Dead in the eyes. Wormtail can't help but feel glad he himself escaped that fate.
Remus is there, too. Peter can't remember ever being this scared, would rather face Voldemort than these two people who used to be his friends.
They turn him back into a human. His voice is hoarse. He knows what they will do to him; they will kill him because he killed James and Lily. He begs with Harry, with Ron, with the girl, but it is no use, and he knows it -
Harry saves (sentences) him. The dementors will have him, will take his soul, and he wishes very desperately that he'd just been killed instead.
But it is a full moon, and Remus has forgotten, and in the confusion Peter transforms.
He is a rat.
He escapes Moony and Padfoot and Prongs's son. They will not find him, not in the dark, not with a werewolf to deal with.
He runs (scurries) away.
Wormtail survives on his own, finds his master, brings him back. He takes Harry's blood.
The war begins, and continues, and it like the first one, only Sirius and James and Lily are dead. He is a rat. He has no friends.
He stays with Snape (Snivellus), and then the Malfoys. He hides.
Harry and Ron and Hermione are caught; she is tortured by Bellatrix. There is a crack from the cellar, a Disapparating kind of crack, and he goes down to check.
They are escaping, and Peter could kill them, could kill his friend's son.
He can't.
(He can't kill Harry)
And so he dies instead, strangled by his (Voldemort's) silver hand. He can't breathe, he is going to die, he knows it, and Harry (kind, forgiving Harry) tries to save him, the Secret-Keeper, the betrayer, the reason his parents are dead.
Harry's (Lily's) eyes are the last thing he sees.
A/N: Anything and everything you recognize belongs to J.K. Rowling. Thanks for reading!