Peter was a more prominent player in the penultimate part of the Divergent trilogy. In Allegiant, however, he appears sad and defeated all the time. He's conflicted about his past choices (dude, who isn't?), but he can't find a way to move past them. It doesn't help that everyone hates him for what he did, including Tobias, who says, "I cannot forget what Peter did, because he has not changed" (33.25).
But Peter really wants to change. He says, "I'm sick of doing bad things and liking it and then wondering what's wrong with me" (46.17). So he decides to drink the memory serum and become a different person. Tobias helps him erase his identity in less time than it takes to drop an Etch-a-Sketch.
But doesn't that count as taking the easy way out? Why does Tobias, who hates cowards, assist Peter in doing this?
In the end, new-and-improved Peter finds himself a boring office job in Milwaukee. Maybe that's like penance for all his bad deeds?