JK Rowling says the following about Peter Pettigrew: He's weak, fundamentally weak. Pettigrew is a very weak character. He's not someone I like at all. He's a weak person and he likes to gravitate to people who are stronger. JK Rowling at Carnegie Hall 10.20.07 Who was Peter Pettigrew? Peter Pettigrew (c. 1960 – March, 1998), also known by his nickname "Wormtail," was a wizard and son of Mr and Mrs Pettigrew. He began attending Hogwarts School of Witchcraft and Wizardry in 1971 and was sorted into Gryffindor House after the Sorting Hat pondered over which house he belonged in for over five minutes, a true Hatstall. During his years at Hogwarts, he became one of the Marauders; he was best friends with Sirius Black, James Potter, and Remus Lupin, and together they created the Marauder's Map. During the First Wizarding War, Peter was a member of the Order of the Phoenix, but became a spy when he defected to Lord Voldemort. He was made Secret-Keeper for the Potters when they went into hiding with the use of the Fidelius Charm, and betrayed James, his wife Lily, and their son Harry to Lord Voldemort. After Voldemort's fall, he faked his own death and framed Sirius for betraying James and Lily, as well as for his own murder and those of the twelve Muggles he killed during his escape.
Why did Peter Pettigrew choose Lord Voldemort and betrayed his closed friends?
Peter didn't chose Tom Riddle, Lord Voldemort chose Wormtail. A choice assumes he felt he actually had a choice. It could have been that Tom Riddle choose Peter. He would have appealed to his vanity and popularity at a time when most of his friends were going off and having lives. James, Remus and Sirius were great wizards, going off doing great things and here he was, no where near their caliber. He would have left Hogwarts to do far less glamorous tasks than the rest of the Marauders. Imagine his envy? Peter Pettigrew was a nondescript wizard in the world he lived in. Never a leader, always a follower. His type of character is one of self-servance and self-protection. Pettigrew only helps or befriends those who might offer something he needs. Most of the time, Pettigrew is content to tag along with the perceived leader of the group.In school, he tagged along with James Potter, Sirius Black, and Remus Lupin. This provided marginal protection, along with a certain sense of belonging to an elite group. These young wizards had a few secrets they were keeping cloaked, along with an advanced magical ability. Peter would have been quite susceptible to Tom Riddle's charm and promises. An affect of many charismatic and manipulative leaders, is not realizing how deep you are until it feels too late to turn back. Lord Voldemort's pitch was superiority over muggles and half-bloods, an inferior feeling Peter would have been entranced by such a pitch. Once Peter tasted that power, the power Lord Voldemort offered, it's doubtful he'd settle for his old friends.
When Lord Voldemort sat wondering who, among those who would stand with Dumbledore, could he pick-off and win over, who would act as a spy, who would they trust? Who was weak enough to control and manipulate he know that we had found a ready and willing participant in In Peter Pettigrew: someone looking to feel more important than they were. Pettigrew's betrayal of the Potter family was simply the next step to solidifying his new position within Voldemort's growing empire of evil. Pettigrew might be a somewhat mediocre personality, but he's adept at survival. This would have worked out in his favor, had the Killing Curse not rebounded upon his dear leader of choice. Pettigrew knew he was exposed, and he knew the exactly how to mitigate his damages. He did what self-serving cowards are very skilled at doing: shift the blame and focus to a convenient scapegoat. Kill two birds with one stone, and make that scapegoat the very person who could wreck your life--Sirius Black. It was all too easy to create the diversion, set Black up as a murdering turncoat, and escape to a life of hiding in the dirt like the rat he is. The only flaw in the plan is time. Time has a say in all matters, and is persistent about it. Pettigrew had no way to know the future, much less control it. It really was a matter of which catalyst resurfaced first; Black or the Dark Lord. Either way, the shadowy path of Pettigrew's life journey had only one option which was to wait and see. This is what happens when you stray from the well lit sidewalk.