User blog comment:27finnsprincess/Finn may be the Tragedy/@comment-4889451-20121020110219/@comment-5252865-20121021150611

I would say that :

Billy asked Finn to get a spell or some thing that was writen on a board somewhere in the candy castle, PB sees them and asks them why are they taking this. Finn says they need to get it to Billy, PB is shocked and rides after them and tries to tell them to not do this. At this point the picture takes place as Finn hears PB saying to them that Billy is evil. And that she was the one who sealed him in the cave to save land of ooo. Best plot twist I can think of :p