I'm going to steal my exroommate's calculator just before her math final exam
Thats rude! Why would you steal her calculator just before a big exam?
I've been sick of her fussing about her math homework.
I can see how you might be frustrated.   It is not right to ruin someone else's chance to pass a test.  You could damage their education and make an enemy.
Who does math or cares about a stupid test.
Obviously your room mate and you seem to know this since you're intentionally stealing her calculator. You seem to be aware that she needs it and that failing would be bad for her. Return her calculator before she finds out and blames you for failing the test.