So, I was looking through the Did the Sorting Hat Mis-sort Key Characters?
and I noticed almost everyone found problems with Peter's sorting. According to JKR, the Sorting Hat's choices were impeccable, so I guess Peter was somewhat brave. It is in my personal opinion that Peter, from what we know of him when he was an adult, was best suited to be a Slytherin.
I don't think Peter was a Slytherin as an adult because he ended up as a Death Eater. I am basing my opinion based on Slytherin's qualities. Slytherin's are considered cunning and ambitious and, according to JKR in her interview with Pottercast, they tend to have a sense of self-preservation. All of these qualities, IMO, describe Peter.
Peter was cunning enough to do many things for Voldemort. He successfully managed to fool everyone in the Order. Some speculate that Peter played a role in the mistrust Sirius and Remus had against each other. If that is true, then this strengthens my belief that he's cunning. He must have been subtle in convincing either of them to go against the other. Another example that supports my belief is that Peter managed to make Sirius look guilty for all the things he did. He was cunning enough to make it seem like Sirius betrayed the Potters and killed him along with twelve other people. Not to mention, Peter figured out how to hide from the Death Eaters and still be in touch with the Wizarding World.
Peter was pretty good at keeping secrets. There isn't any canon that suggests Peter gave huge hints that Remus was a werewolf, so he must have been good at keeping that secret from others. Peter's greatest achievement in keeping secrets is when he fooled everyone into thinking he was on the Order's side. There isn't anything that says being secretive is a Slytherin trait, but I would associate it with Slytherin the most most out of all the houses. JKR said she based the houses loosely off the four basic elements, with Slytherin's element as water. One of the qualities of the water element
is being secretive. Therefore I do think it is a trait that describes Peter and Slytherin, but I understand if others disagree.
Peter's ambition is very similar to the next trait, self-preservation. Peter's main ambition is for selfish reasons. His main ambition throughout his life was , in my opinion, to keep himself safe.
This is a very dominant trait in Peter's personality, I think. Peter chose to become friends with the Marauders since they were popular and could remain "safe." Peter chose to betray his friends and the Order by joining Voldemort because he thought that was the safest thing for him to do. He chose to return to Voldemort because he was scared of his former best friends. One of the only times Peter didn't do something for his own selfish reasons was when he let go of his grip on Harry's neck. That wasn't selfish, that was remorse. If Peter didn't die and Voldemort got wind about Peter's betrayal to him, I think Peter would have suffered considerably.
I'm not trying to say that Peter wasn't brave, but I think Slytherin described him better, based on his behavior that is shown in canon.