Moral Dilema

We had an exam this morning-multiple choice, 80 questions. When I finished, I walked by one of my roommates. She had a scrap piece of paper where she was writing all the answers to the test. I almost told the Sgt in charge of testing that he might want to check it out, but I hesitated. She could have been using it to organize her thoughts. Plus she put the paper away when they went over test answers. Later today, I found the paper crumpled on top of her desk. The thing is, taking test answers out of the classroom is an integrity violation. Not only that, but knowing about the test answers and not bringing it up is also an integrity violation.

I told my roommate and told my Staff Platoon Commander. My roommate pretty much hates me. I'm not trying to kick her out of the Marine Corps, or make others think less of her (word gets around), but I had to act. I had a moral obligation to report it. So, why do I feel like I did something wrong?