Why does Douglass compare Master Hugh Auld to a pirate?

A pirate is one who commits wrong, but sees that his greater end (to gain fortune or capture people with the desire to gain money) is more important than any morality that he might be violating.  Master Hugh has somewhat seen the wrong with slavery, but he avoids facing his knowledge and continues to commit his "wrong" because of the greater gain that he sees the slaves bring him.