I'm thinking of keeping some of my roommates money when she moves.
Why are you going to steal her money?
I want to make fun of her. You know, she's got this rich dad. It would be hilarious.
No matter if she's rich or not, theft is theft and you will be held accountable. You are breaking her trust by stealing her money which is rightfully hers. Can't you make a harmless prank or joke instead to see her off?
I have to admit,  it's really entertaining to me. Also, my roommate deserves a lesson.
Nobody would ever trust you if you stole her money and people got to know about it. You have no right over your roommate's money. You should give it back to her.