I'm not going to replace toilet paper when my roommate didn't before me
Why are you going to do that?
Because I hate my roommate's laziness.
You both are wrong in this situation. You should change it and so should they. You can set an example by doing this.
Why should I do this when she won't?
Because it is better to be the bigger person in the long run. Stop being petty and replace the toilet paper.