
In the book Deathly Hallows, Harry reminded Pettigrew of the life debt that he owed to Harry, causing the hesitation.

This also allowed Pettigrew the opportunity to escape and return to Voldemort.

The moment was set up in Harry Potter and the Prisoner of Azkaban, when Harry chose to stop Sirius Black from killing Pettigrew and allow him to live.

While the movie producers may have been trying to protect younger viewers, they did a disservice to the story arc for Harry and Wormtail.
