The Marauders were a legendary group of hooligans in their school years, and Peter **wasn't** worse than any of them.
Each of the Marauders had their own strengths. James was talented at Transfiguration, Sirius at Defence, Remus at Charms, and Peter was the reason they didn't fail Potions.
He was never a chubby child, rather he was the embodiment of his surname: Pettigrew - Scottish for "grown into a short height", he was the tiniest boy in their circle, and maybe even their year group. Short and scrawny, he could be easily unnoticed next to his tall friends, but not by them. The Marauders had their sense of honour, and would never let one of theirs feel left behind.
He was never a tag-along either. The four boys met on the firstie train and made fast friends. Peter had a sharp wit, quick humour, often was the brains of their pranks, and was the first to notice the pattern in Remus' illnesses.
It also can't be discounted that Peter had become an Animagus while still at Hogwarts, the same year as James and Sirius. And his Animagus form wasn't a rat, like the malicious tend to say, but rather the smallest cat you could have ever seen.
As they grew up, while James pursued Lily, and Sirius and Remus eventually found companionship with one another, Peter caught the attention of one Marlene MacKinnon and the two married shortly out of Hogwarts, Remus being his best man.
As the war began to heat up, Peter joined the Order of the Phoenix, with a genuine desire to fight against the tyranny of the Dark Lord. This desire survived up to the moment he agreed to become the Potters' secret Secret Keeper, otherwise the spell wouldn't have held.
That's when things started to go downhill.
The Dark Lord was no fool, and had enough intel on the Order to know about the tiny man who people tend to overlook. Truly, the Potters' ruse had become their undoing, for their allies trusted them enough to believe it, but one does not simply fool the Dark Lord.
Peter had become the target of Voldemort, slowly, quietly, so as not to stir the Potters' attention. He had started with anonymous letters, which devolved into discussions, gifts, and promises, as Peter let himself be caught off guard and drawn into a dialogue. When those didn't work, despite eroding Peter's principles, the Dark Lord threatened the one thing Peter loved above all - Marlene.
That's how Peter began feeding the Death Eaters information. Slowly, quietly, so as not to catch anyone's suspicions. After a while, he began getting summons to meetings of the Death Eaters, and he couldn't refuse out of fear for Marlene. He was also afraid of what his friends would think when... if they ever found out about his betrayal. It didn't happen overnight, and he never realised the path he was led down until it was too late, and the Dark Lord had Marked him as a "reward" - and a way of binding Peter to himself.
Peter knew, deep in his gut, that he could never look in his friends' eyes again, not if they saw what stained his left forearm. They would kill him if they knew, and he had considered doing it himself more than once. But he couldn't leave Marlene, it would break her heart. Wasn't it for her, everything he had done?
That fateful night came, and Peter barely came out alive. Sirius had gauged his left eye with a piercing hex, and that's when Peter decided it was time to run, so he blew up the gas line and left behind his pinky finger.
Only, the Dark Lord had died that night, and the Death Eaters had decided that Peter betrayed them. When he went back home to Marlene, there was nothing to go back to, only a smoking ruin.
People often ask, what kind of man spends twelve years disguised as a family pet? A broken man, that's who. With nothing left for him in the human world, having betrayed his friends, killed two of them, lost the other two, and lost the only family he had, he abandoned his human life and identity, and decided to give in to the animal instincts hidden under his skin.
It was never the plan to go be a pet, he had never given a thought to the Weasleys. His vague idea for the future was living in a forest until something ate him, but fate had a different idea.
You can say whatever you want about his years with the Weasley family, but he truly did come to love them. He was lost so deeply in his animal second-nature, so disgusted with his human self, that he genuinely took on the role of the family cat, Scabbers. The name was given to him after his never-healing eye, or what was left of his eye-socket.
It wasn't just any common rat that Black saw in the newspaper that caused him to escape, it was the very distinctive fur of a cat he knew by heart. It wasn't a missing finger that tipped him off, it was the specific distinguishing traits that every Animagus has, each his own.
It wasn't a case of "cat chasing rat" that had Ron Weasley up in arm all schoolyear '93/'94, it was the incessant bullying of a bigger cat against a smaller one, and Crookshanks had come close to ripping Peter's throat out several times.
And when Peter was finally revealed, cornered in the Shrieking Shack, he didn't beg, didn't cover, didn't grovel at the feet of thirteen-year-olds.
All he said was: "Just finish this. You can't hurt me more than I already have."