English is not my mother language so i´m sorry for any mistakes :)
Everybody assumed that Peter Pettigrew gave away the secret to the Potters fidelus because he was afraid of the Dark Lord.
Of course, this was true, but it wasn´t the only reason.
You see he held quite a grunge against James Potter and he had waited for a long time to get revenge.
It started out in 4 year when they discovered that Remus was a wanted to keep him company so they decided to become (quite illegally) animagus. They trained and trained and secretly Peter hoped that he´d become a lion or some other majestic animal.
He was the smallest in the group of 4 and always more of a background player so it wasn´t very surprising that he wanted to be something big and intimidating.
So after they finally made it during fifth year Peter was devastated to find himself once again in the role of a small unassuming persona.
While everybody else at least coud keep up with Remus form he was more or less left behind.
That hurt of course. The others found it funny and kept making jokes but he felt like he didn´t belong to the group and it made him sad because nobody likes to be left out.
What was worse in his option was the nicknames the others in specifically one James Potter decided to give each others.
Prongs for James.
Padfoot for Sirius.
Moony for Remus.
Wormtail for Peter.
He was not amused. What kind of nickname was Wormtail? When he tried to protest in the common room James overrode every argument of his and stated firmly that Wormtail was fitting and this was the only name he would call him from now on. Sirius followed of course and while Remus gave him a excusing glance he didn´t say anything against James decision.
Unfortunatly James was overheard by Melissa Pittlegrind - the gossip queen of Gryffindor. When she heard how "fitting" Wormtail was for Peter she naturally assumed something else than what James ment. It took one day for all of the Gryffindor Ladys to hear the news and three more days for news to travel to the other three houses.
It didn´t help that once James heard the rumor he started calling him by his nickname not only in private but in public too.
Soon everybody followed suit and Peter was confronted with giggling girls everywhere he went. A small part of him liked the attention everybody gave to him but mostly he was ashamed and tried to hide from girls as often as possible.
Jame and Sirius felt it was a big great prank and told him to loosen up. It was fun for them to loudly start giving him flyers of advertisement for "certain" enlargement pills and encourage him to go to Snivelus for potions which could "help" him.
After he tried to ask out Melody Winterstein in his sixth year and all she did was look at him horrified and tell him that she liked men with something down there, thank you very much, he officially started hating James for giving him this stupid nickname and swore revenge for all the humiliation.
If there was something he was good at it was concealing his feelings towards James and sit and wait for the perfect opportunity to destroy him.
So when Sirius asked him to take over his position as the secretkeeper for Potter he sensed his chance and took his pleasure in giving Voldemort the address of the family.
End.
I hoped you like the one shot. Its nothing special but it´s my first one in english so i´d be happy if you left me a review :)