Harry Potter and the Prisoner of Azkaban plot hole?

It's not a plot hole, Sirius wasn't even guilty. It was just a comment made by Hagrid wasn't it? That there wasn't a wizard that went bad who wasn't in Slytherin. However, this doesn't mean that everyone in Slytherin will go bad. Also, this question would be better asked about Peter Pettigrew as he was the one who betrayed the Potters. However, I think that was just due to his weak character and his need to please people and to be sycohphantic .