Peter's choices towards the end of Fringe copied Walter's original choices towards the beginning. Fringe is the story of Walter, Peter, and their friends coming to with the consequences of Walter's nearly world-ending actions. Walter's experiments many years before had created a hole in reality. The Fringe Unit was created to deal with the consequences of that breach. Olivia needs both Walter and Peter and facilitates their reunion. At first, their relationship is rocky, but they grow closer over time. Initially, Walter resents Peter for abandoning him in the mental hospital after his experiment killed someone. At first, Peter resents Walter for pretty much everything.
The main reason that Walter and Peter have so much friction is that Peter can never fully understand the reason why Walter has made the fringe choices, especially concerning Peter. Throughout Fringe, Peter realizes that he and Walter aren't all that much different. And once Peter's daughter Etta is born, Peter begins to grow closer to his father. He realizes that love and family are compelling motivators and can inspire all sorts of choices one wouldn't make in any other circumstance.
Fatherhood Drove Peter and Walter's Choices In Fringe
Fatherhood is central to the story on Fringe. Through a window between universes, Walter saw that alternate Peter had the same illness that killed Walter's son, prime Peter. Walter also saw that Walternate had unknowingly found the cure. Wanting to save alternate Peter, Walter crossed to the other universe. He initially just wanted to talk with Walternate, but Walter brought Peter back to this universe. This choice to be a father to Fringe's alternate Peter nearly led to the world's ending. Walter does everything because he loves his son and is willing to sacrifice reality to save him.
Peter and Olivia are separated from Etta by the Observers coming to Earth and searching for her for many years. Shortly after they find her, Etta is killed by Observers. Peter is broken by his daughter's death and installs Observer technology in his body to help defeat them. However, he begins to become as emotionless and cruel as the Observers. Olivia is terrified by these changes and begs Peter to remove the X-files-like technology. Fringe's Peter refuses until Olivia invokes his love for Etta. Finally, he realizes that becoming an Observer would betray his love for his daughter and decides to fight as a human for the future of humanity.
Why Peter's Fringe Story Had To Copy Walter's Origin
Peter ends up fixing what Walter broke when Peter saves his daughter. He and Olivia, motivated by their love of Etta, create a stable timeline without Observers. Instead of dying young at the hands of the Observers, Etta presumably lives a full life. Peter risks it all to create that future, and Walter, who understands all too well how the love of a father drives his own choices, helps Peter. The final piece to stopping the Fringe season 5 invasion of the Observers involves Walter going back in time and staying there. This resets the timeline from 2015 onwards, which means that, sadly, Walter disappears from reality in that year.
Throughout Fringe, it's clear that most of the decisions Peter makes are influenced by Walter. Initially, Peter doesn't understand the sacrifices that Walter has made for him and sees him as an immoral experimenter who does nothing but cause Peter misery. Peter doesn't visit his father in the mental hospital until Olivia makes him, and he continually refers to Walter by his name rather than calling him Dad. However, Peter grows to respect and love Walter and understands that Walter loves and respects Peter in return. The moment in the Fringe series finale when Walter walks into the past while leaving the future to Peter is a heart-wrenching full-circle moment for both fathers.
