I don't see any problem (morally) with stripping DRM from a rental in order to move it onto a device that wouldn't usually support it. However I do feel you should delete the book when the rental period is up. Keeping it is wrong both legally and morally imho.
With something like Amazon where you pay to rent the book, I think there's no question that you should delete it when the period is up, whether you've nearly finished it or not. You paid for that time limit only.
With a library book, I'd be a little more lenient, if you're on the last few pages and need it for just one or two days more yet someone else has it reserved, I'd say finish the book then delete it. If your another week or two away, then perhaps you should delete it and re-rent it later. You certainly shouldn't check out any more books until you've deleted it though. That way, you're simulating not returning the physical book, just there's no late fee to pay, but you don't get another one until you have returned it
Publishers want digital library books to "decay" with usage just like physical books, it's only fair that digital books can be returned late too
