He betrayed his best friends in the worst way imaginable, he sold James and his family out to Voldemort which led to his and his wife's death and simultaneously condemned their 1-year-old child into being an orphan who had to suffer through years of abuse.
He killed 12 innocent people and framed his other best friend Sirius Black for the crime and betrayal he himself committed, who in turn was unjustly condemned of having to spend 12 years of his life in Azkaban and the rest of it on the run.
He helped kidnap and torture his best friend son of 14 years who previously saved his life from certain death, and went on to kill an innocent boy in the process, all in the effort of bringing back to power the evilest wizard to have ever lived who destroyed and took countless of lives including those of his friends who would’ve done anything for him.
Now tell could you simply forgive stuff like that?! It wasn't as if Peter Pettigrew went behind his friends back and merely embarrassed them, he completely destroyed their lives and stumped on their friendship with his bare feet as if it meant to nothing to him, just to safe his own skin.