Peter Pettigrew was once a regular wizard just like the Weasley's but he had one difference, Peter was an Animagus. a wizard that was able to transform into a chosen animal for as long as he or she wanted, kind of like a werwolf but able to control it. His story started when he became friend with some other male wizards called James Potter, Sirius Black and a werwolf named Remus Lupin who all loved to play pranks on the other students and teachers in the school and where always up to no good they would run around the castle late at night and set tricks up so the next day they would be awoken by the shrieks and yells of their classmates when they had been attacked by a Dr. Filibuster's Fabulous Wet-Start, No-Heat Fireworks or a rouge dung-bomb.
The four boys all became so close they all decided to become Animaguses so when Remus turned into a wolf they transformed into animals so they could be with him and stop him from killing other wizards James Potter transformed into a deer, Sirius Black changed into a large hairy black dog and Peter Pettigrew shrank to an ugly fat rat.
After Hogwarts they all went on with their lives and did what all the other wizards did until, an evil dark wizard named Voldemort also known as he who should not be named started Murdering innocent wizards and even muggles (non magical humans) he then targeted James Potter and his wife Lilly who had recently had a child called Harry. So when all their friends found out about this they all volunteered to be their secret keeper who was the only person who was able to know their location and whereabouts the Potters accepted this proposal so James chose his loyal friend Sirius Black until they realized that is was too easy for the dark lord to find Sirius so they chose someone who Voldemort would never guess to be their secret keeper, Peter Pettigrew. So it was all sorted Peter was the Potters secret keeper and was the only one who know where they where and what they where doing. But no-one knew that Pettigrew had turned to the dark side and was serving the dark lord and obviously told him where the Potter family was and Voldemort killed them not soon after except from Harry Potter the Potter son who was the only one who had ever survived Voldemort's killing curse which backfired on him and went into hiding to regain his strength.
When the news started to spread everyone thought that Sirius was still his secret keeper and he was the only one who knew that Peter was their recent secret keeper and went out to find him and after a long search he found him in a muggle street where he wanted to kill him but as all the other muggles where in his way he was unable to use his wand and murder him but Peter then realized that if he stayed in the wizarding world in his human form he would be a dead man so he had an idea a very evil idea he thought, that if he killed all the muggles in the street and transform into a rat so everyone would think that Sirius did it and He did, he killed all the people in the crowded street with an unforgivable curse and turned into Scabbers and Sirius was soon taken away to Azkaban prison for a life sentence.