It is guilt (and I'm not even Catholic!) If someone got in trouble for decrypting library books and keeping them, even though I didn't get caught, it would bother me. I want to be able to use the library, guilt-free.
I have already read, AND DELETED, 2 of the 6 books I checked out last night. So you can tell what kind of trashy books I read. Now, I wish we could turn them back in before the 3 weeks are up - 6 books isn't enough for 3 weeks! Of course, I could read some "good" books in between.