It is revealed that inside each Prinny is a soul that has sinned. In order to be reborn, each Prinny must either perform a number of good deeds or collect enough money that is equivalent to the severity of the sin(s) committed.
(Laharl is accused of stealing the Demon Tome when nobody was looking.)
Laharl: I would never do something so despicable!
Etna: But he'll rob you blind when you're away!
Laharl: (Laughing to Koganesky) You want me to off your dad?
Title: A Solomon-style Judgement at the Dinero Castle?!
Solomon was a king in one of the stories in The Bible.