It's also interesting to note that Huck's realization is not presented in some touchy-feely "we are the world" sugar-coated PC moment.
Instead, because of his upbringing Huck actually believes that the right thing, even the godly thing, is to return Jim (the runaway slave) to his "owner". In one of the most marvelous passages in American literature, Twain has Huck decide to protect Jim *not* because he's piously doing the "right" thing, but even though he believes it's the *sinful* thing:
[After writing -- but not yet mailing -- a letter revealing Jim's hiding place] I felt good and all washed clean of sin for the first time I had ever felt so in my life, and I knowed I could pray now. But I didn't do it straight off, but laid the paper down and set there thinking -- thinking how good it was all this happened so, and how near I come to being lost and going to hell [by helping Jim escape]. And went on thinking. And got to thinking over our trip down the river; and I see Jim before me all the time: in the day and in the night-time, sometimes moonlight, sometimes storms, and we a-floating along, talking and singing and laughing. But somehow I couldn't seem to strike no places to harden me against him, but only the other kind. I'd see him standing my watch on top of his'n, 'stead of calling me, so I could go on sleeping; and see him how glad he was when I come back out of the fog; and when I come to him again in the swamp, up there where the feud was; and such-like times; and would always call me honey, and pet me and do everything he could think of for me, and how good he always was; and 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; and then I happened to look around and see that paper.It was a close place. I took it up, and held it in my hand. I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. I studied a minute, sort of holding my breath, and then says to myself:
"All right, then, I'll go to hell" -- and tore it up.
It was awful thoughts and awful words, but they was said. And I let them stay said; and never thought no more about reforming.