Jim's "What's trash is trash" speech, which utterly dismantles Huck's amusement at pulling a vicious prank on him. From that point on, Huck (and the reader) treats Jim with much more respect.
There comes a point at which the Duke and the King, a pair of treacherous traveling companions, have turned in the escaped slave Jim, leading to his recapture and imprisonment. It falls to Huck to write a letter to Miss Watson, telling her that her escaped slave has been recovered, and how and where she might retrieve him. At least that way he'd be back with his family. On the one hand, the prevailing social order of the antebellum American South says that this is nothing more or less than what is ordained by God, as is every other aspect of the 'peculiar institution'; in this formulation, Huck's aid in Jim's escape - legally, stealing him - is a grave and possibly mortal sin, and aiding in his recapture serves as expiation. On the other hand, Huck and Jim have been through a lot together; no matter how hard Huck tries, he can't seem to think of Jim as anything else but his best and closest friend, and how they've stood by each other through it all. Then his eye falls on the letter he's just written. Huck's decision:
"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. I shoved the whole thing out of my head, and said I would take up wickedness again, which was in my line, being brung up to it, and the other warn't. And for a starter I would go to work and steal Jim out of slavery again; and if I could think up anything worse, I would do that, too; because as long as I was in, and in for good, I might as well go the whole hog."
Pay special attention to the emphasis. Huck doesn't say, "I'll go to hell". He says, "I'll go to hell." He isn't being sent there - he's marching there defiantly.