I don't own Harry Potter. I do worship reviews.
Before he does it, he actually walks to the library and picks through stories of famous traitors through history. Benedict Arnold. Marcus Brutus. Caius Cassius. Judas Iscariot. He imagines his own name beside them someday, black and white letters in a narrow column that can't possibly tell—even Brutus thought he was doing the right thing.
Before he does it, he visits Sirius. Sirius' face is pale and he looks like he hasn't been eating right and something in the way he speaks lets Peter know that he isn't exactly welcome. But he pretends to ignore it, and stays until dark, taking a sort of pleasure in the way Sirius' jaw clenches as he serves tea and cookies. They talk about Quidditch and the new shop in Hogsmeade and after the war—Peter just wants to shout that after the war isn't coming.
Before he does it, he agrees to babysit Harry for a day. Lily and James are going to Hogwarts to talk to Dumbledore about something. Harry is asleep when he arrives, and he feels fidgety alone in the house. He paces back and forth, wincing when his cloak brushes against the back of the couch. He's relieved when he hears Harry crying. They can't go outside, but they spend the afternoon baking brownies and eating the batter. He reads Harry a book and sings him a lullaby as he falls asleep in his arms.
Before he does it, he pulls his box of old Hogwarts things out from under his bed. He hasn't grown since seventh year, and he slips on his school robes and Gryffindor scarf. He stands on his back porch, watching leaves roll by, and flips through his photo album, his thumb gently rubbing against the well-worn edges. He sees himself in Hogsmeade and in the Common Room and the Quidditch stands. He sees himself with Remus and Lily, Sirius and James. But he can only look for so long before his eyes water (from not blinking) and he slams it shut and hurries into the house.