guys... try to be in peter's pov. his friend got humiliated in front of him and the letter that sent to him before her death was about her hate to philip and how he tricked her. peter didn't get to meet jennie for days after the incident in prom party so there's still something missing in the story. i think it's make sense if peter want to take a revenge for her. we are saying that his decision is not make sense cause we see it from our pov as a reader who knows everything. peter didn't know what actually happened and how philip is actually innocent. he just said that he didn't wrote the letter and that's it. they still got humiliated by his friends and who knows what happened with jennie after peter left her.