Guys, I think you're forgetting something about PB.
Namely, she isn't human.
Not only has she conquered death, her lines in Too Young imply that she can create "candy matter", which implies everyone currently in the Candy Kingdom could be her creation.
She doesn't perceive time in a linear fashion, but rather based on how "old" her body currently is; she thinks everything that happened when she was thirteen was literally five years ago, despite the fact that she's almost always been eighteen. Everything that happened from season one till now is, for her, as fresh as yesterday.
She also apparently doesn't have parents.
All things considered, I'd consider her less "evil" and more "alien". Marceline and Ice King went insane, FP is a force of nature, but PB apparently developed her morality by herself. All things considered, it's amazing she's as nice as she is. It's not that she pretends to be nice while secretly being evil, it's just that she doesn't think being a Mad Scientist is wrong as long as everyone is happy.
Remember when Finn went crazy with the glasses? PB only thought there was something wrong so long as her people were frightened. When it turned out that they had enjoyed themselves, she instantly changed her mind.