We don’t really know much about Peter Pettigrew, do we? We know he was friends with James, Sirius, and Remus – best friends, though he was described as always trailing behind them, never quite as smart, or charming, or good looking. But still. They loved and trusted him and it’s assumed he did the same. But he abandoned them, throwing away his friends and their families on the chance to be protected and looked upon with favor. It was the most cowardly thing he could have done. I don’t know his blood status – HP Wiki lists it as either pure blood or half blood – but his friends were instrumental in the movement AGAINST Voldemort, and he clearly felt that put him in danger and chose to defect.
Sometimes, I want to feel bad for him. After all, Voldemort calls him by the name his BEST FRIENDS called him – the best friends that he betrayed, and helped murder, put into prison, and make a penniless outcast. I just…can’t. No, he wasn’t as talented or as smart or as brave as his friends. And yes, it was a scary, scary situation to be in (particularly if he was in fact a half-blood). But he had other options. He could have simply run and hidden, stayed aside from the fight. But he didn’t. He actively betrayed his friends, who trusted him enough to put their lives – and the lives of their infant son – in his hands. And he takes that trust, and spits on it and grinds it into the dirt with his heel in the HOPES that his treacherous loyalty will be rewarded under Voldemort’s reign. Later, Harry saves his life, and Wormail repays that kindness by continuing to work for Voldemort and helping him try to kill Harry.
That sort of selfish, short-sighted, treacherous behavior might be human, but I think he’s more at home as a rat.