What is Huck's understanding of human nature in The Adventures of Huckleberry Finn?

Huckleberry Finn's view of human nature is that humanity in all its levels is fallible and not to be trusted at face value. Huckleberry's father was irresponsible to the extreme with Huckleberry and with Huck's education. A drunkard and violent drifter, he left Huck to fend for himself in more ways than a few. As a result, Huck had no academic or moral upbringing to speak of.

Further, when Huck is taken in by Widow Douglas who gives him some education along with moral and religious training, his suspicion of others' intentions and rightness (resultant from his upbringing under his father's hand) leads him to question and reject some of what he hears, partly because he hasn't been taught to accept and partly because his circumstances forced him draw conclusions for himself based on what he perceived and experienced.

The one exception to Huck's view is Jim, whom Huck sees is in worse straits than he himself is. Or perhaps part of the reason Huck is persuaded to act so abominably toward Jim later in the novel is that Huck feels deep inside that Jim is fallible and not to be trusted as well--and the misadventures of a boy's untempered imagination can therefore take sway over Huck's better judgments.