What white attitude of the time does Twain attack in Huck's conversation with Aunt Sally?

There is bitter irony in Huck's story about the steamship cylinder exploding. Huck concocts the tale as an excuse for arriving in town so much later than expected, and when asked if anyone was hurt, he replies "No'm, killed a nigger." Aunt Sally is relieved to hear that no white people where hurt or killed, and does not care that a black person died. In the beginning of the book, the reader could easily attribute racist attitudes to the culture and time, forgiving the speaker for his or her ignorance, but after being introduced to Jim, the reader is unable to maintain that distance. Thus, it is surprising to hear Huck make such a racist and hypocritical off handed comment, but perhaps he is simply speaking in a way he thinks Aunt Sally would relate.