The point of calling Animal Control vs just taking the dog, (other than any criminal charges that could have potentially been brought against you) is that your aunt would have a history of abuse with the authorities and would most likely have been barred from owning another dog in your city/county. She could conceivably still go out and purchase another animal despite the restriction, of course, in which case you would again call Animal Control and the dog would again be removed from her custody.
In my opinion, you have not only committed a crime by removing the dog from your aunt's home without previous permission, but you have allowed a cycle of abuse to continue.