Aunt Sally's reaction to the black man being killed reflects society's attitudes towards blacks very well. It shows that society did not care about black people and really did not see them as people in any real way.
When Aunt Sally hears about the steamboat accident, she is very worried. But when she hears that only a black man was killed, she is quite relieved. In fact, she just about comes out and says blacks aren't people. She says
Well, it's lucky; because sometimes people do get hurt.
This reflects the attitudes of her society where slavery was acceptable and even seen as good.