How is conscience a theme in The Adventures of Huckleberry Finn?
Huck Finn himself is often led by his conscience instead of by societal rules. He believes that there are certain moral obligations that are above and beyond the restrictions of society. This is shown powerfully in the scene where Huck decides to steal Jim out of slavery; he thinks about how society keeps Jim and other slaves from freedom, but also how he has become a sort of person that he despises:
...a person does a low-down thing, and then he don't want to take no consequences of it... The more I studied about this the more my conscience went to grinding me, and the more wicked and low-down and ornery I got to feeling.
...at last I struck the time I saved him by telling the men we had small-pox aboard, and he was so grateful, and said I was the best friend old Jim ever had in the world, and the ONLY one he's got now...
(Twain, The Adventures of Huckleberry Finn, gutenberg.org)
Here, Huck's conscience fights him on two counts: first, he is convinced that his place in society, and the ill-will he will receive from his friends and neighbors, is worth betraying Jim's trust for. He believes that he is morally required to betray Jim because that is what society expects of him. However, he then goes on to remember how deep his friendship with Jim actually went; Jim, used to being treated like property, was grateful to Huck for simply treating him like a human being, and so Huck begins to feel even worse about going back to that original expectation. Throughout the book, Huck's conscience tells him different things about his actions, and he makes his decisions based on the best information he knows at the time.