Although the slimy character was close friends with James during their time together at Hogwarts, Pettigrew ultimately abandoned his friends to join the dark side. In order to prove his loyalty to Voldemort, Pettigrew was obligated to hand over the Potters' hiding location, otherwise he would be killed.
Pettigrew defected from the Order of the Phoenix and betrayed his closest friends to Lord Voldemort, believing that there was nothing to gain from opposing the Dark Lord while expressing his fear of being killed by him.
Peter tends to fall in line with those more powerful than him as he lacks the skills to possess or gain power on his own and therefore reaps the benefit of his associations. That's the explanation given for his turn to the Death Eaters because, at the time of his betrayal, Voldemort was actually winning the war.
Why did the Potters make Peter Pettigrew the secret keeper?
Peter Pettigrew, the Secret Keeper for the Potter family who betrayed them to Voldemort. Sirius Black was originally meant to be the secret keeper for James and Lily Potter, but Sirius thought Voldemort was on to him (he thought that Remus Lupin was a spy), therefore passing on the Secret Keeper to Peter Pettigrew.
Though he valued his own life more than the lives of his friends, Pettigrew felt guilt for betraying Lily and James, but didn't have the courage to admit his treachery or to turn against Voldemort.
Why did Fred and George never see Peter Pettigrew?
There are hundreds of students in Hogwarts, plus ghosts, teachers, House Elves, and Peeves. Unless Fred and George were watching Ron sleep on the map (which would be creepy), they wouldn't have noticed Pettigrew. It's only possible to see a human changed in his animagus form if you know the person is one.
The Harry Potter and the Prisoner of Azkaban book showed that Sirius Black had been believed to be the one who betrayed James since the members of the Order of the Phoenix thought him to have been the Secret Keeper for the Fidelius Charm placed on the Potters' cottage in Godric's Hallow.
'I persuaded Lily and James to change to Peter at the last moment, persuaded them to use him as Secret-Keeper instead of me ... I'm to blame, I know it ...' It wasn't Sirius who betrayed the Potters to Voldemort, but he did persuade them to make Pettigrew their Secret-Keeper.
Peter was taken in by the Weasleys and for twelve years lived with them as Scabbers. One day, while in Azkaban, Sirius asked Cornelius Fudge for his newspaper and saw a photo of Scabbers (aka Peter). Realising he was at Hogwarts, Sirius escaped to come and find him.
Pettigrew hesitated for a few seconds, something his enchanted silver hand interpreted as weakness. This brief moment of mercy caused his silver hand to turn against him, ultimately strangling him to death.
Peter Pettigrew bearing his silvery hand. The potion which allowed Lord Voldemort to return needed three important ingredients: bone of a father, flesh of a servant and blood of an enemy. Pettigrew chopped his right hand off in order to supply the second ingredient.
Dumbledore's lack of belief that Sirius is innocent is also suspect. Sirius was a member of the Order of the Phoenix which fought against Death Eaters, but Sirius also was as close to James as a brother, the Potters even took him in when he ran away from home.
In fact, James outright tells Lily that he's bullying Severus Snape because of the “fact that he exists, if you know what I mean”. James Potter saved Severus Snape from the werewolf prank before the Snape's Worst Memory scene happened. The mastermind of said prank is Sirius Black, James' best friend.
After Pettigrew tried unsuccessfully to undo that moment of weakness, the hand recognised Pettigrew as unfaithful to the Dark Lord and suddenly began strangling him.
He was imprisoned because he was wrongly accused of murdering Peter Pettigrew (AKA Wormtail) in front of muggles; however, Wormtail is the one who actually betrayed James and Lily to Voldemort. Sirius was the original Secret Keeper of the Potter family's location.
Remus fought at the Battle of Hogwarts on 2 May 1998, during which his wife was murdered by Bellatrix Lestrange. Remus was also murdered by Death Eater Antonin Dolohov, during the first half of the same battle. His death was possibly avenged by Filius Flitwick.
Sirius Black and Draco's mother Narcissa Malfoy (née Black) are first cousins because Sirius's mother Walburga Black and Narcissa's father Cygnus Black were siblings. That makes Narcissa's son Draco Malfoy a first cousin once removed of Sirius.
The Malfoys are related to the Black family through Narcissa (a first cousin of Sirius Black, Harry's godfather), which makes Draco a nephew of both Bellatrix Lestrange and Andromeda Tonks.
Its not until lupin discovered that Peter P (wormtail) was still alive that Sirius was innocent and he did not portray the potters or kill wormtail. Therefore Mr weasley thought all along Sirius had the potters killed and though he was out to kill harry because he was the last potter and also he was his grandfather.
Black was able to maintain his sanity due to his knowledge of his innocence, a non-happy thought that the Dementors could not take. It was only this that allowed him to maintain his powers to transform, and what made the escape possible.
As James and Lily's Secret-Keeper, Pettigrew was entrusted with the information of the Potters' hiding location in Godric's Hollow. He had not even been Secret-Keeper for a week when he handed the information over to Voldemort.
If you read the books, then you'll recall no one knew Scabbers was Pettigrew EXCEPT Sirius Black and later Remus Lupin. Why? Because they were the only ones who knew Peter was an unregistered Animagus whose form was a rat.
Scabbers is Ron's rat, but later in the book it is revealed that he is really the wizard Peter Pettigrew who has taken a rat form. The Weasley family adopted Scabbers (the rat) thirteen years ago; he used to belong to Percy but was passed down to Ron when Percy got an owl.
Fred and George Weasley stole the map from Filch's office in their first year at Hogwarts. They had been taken to his office for letting off a Dungbomb and saw a drawer marked "Confiscated and Highly Dangerous". George dropped another Dungbomb, while Fred stole the map from the drawer.
