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[Harry Potter] Why does Peter Pettigrew still serve Voldemort in Goblet of Fire?

I get why Pettigrew initially decided to help Voldemort, and then served under him once Vodemort was back - because he is an immense coward.

But after Voldemort kills the Potters and gets defeated by the little baby Harry, couldn't Peter just ditch Voldemort?  I know, Sirius Black knew that it was Peter so he faked his death and blamed Sirius - but couldn't he have framed Sirius without faking his death?

And even more than that, wouldn't the real cowardly option have been to NOT help Voldemort get back?  Pettigrew doesn't seem like he's a fanatical supporter of Voldemort's message, he only supports him because he's scared of what'll happen to him if he doesn't, but surely that would incentivize him to prevent Voldemort's return?
gpt
Sirius wanted Peter dead. Or at least that's what Peter believed when Sirius cornered him. And Peter knew he could never take Sirius in a fight. So he had to get Sirius locked up and fake his death to make sure Sirius wouldn't come after him. When Sirius escaped Azkaban, Pettigrew knew Sirius was going to continue trying to track him down. And after the events of PoA, he knew Sirius (and Lupin) wanted him dead. So he needed protection.

That said, we know that Voldemort was using snakes and rats as hosts while in incorporeal ghost form, so it's possible that Voldemort just coincidentally happened to run across and use Pettigrew as a host before realizing he hadn't latched onto a normal rat, and Pettigrew decided to stay with Voldemort for that protection.