Young and naïve hero? Check. Suddenly cast into a strange world… like, say, helping a slave escape despite having grown up in a system of rules and morality where that's no different from stealing someone's iMac? Check. Our young hero "falls" out of "sivilization" and comparative wealth into a life on the lam, and he's ready for some adventures.
This raft stuff is awesome. Huck is super-excited to be in the outdoors again, which is to say not getting beaten up by his alcoholic father—or being told that he'll go to hell if he slouches at dinner.
But part of Huck's brave new world doesn't feel so great, the part where he's bucking all the ethical rules that he learned about not stealing your neighbor's property. Only problem is, in this case, that property happens to be a living, breathing human being. With a family.
The whole morality thing really gets tricky when Huck falls in the with the "duke" and "king," who introduce him to all the shady cons you have to run when you decide to take off down the river with no other means of support but your silver tongue. He keeps trying to do right, but the lies pile up.
Huck's crisis of conscience eventually gets so overwhelming that it literally stops him in his tracks: "I most dropped in my tracks, I was so scared" (31). He tries to pray to be a better boy (which would mean returning Jim to Miss Watson), but he realizes that he can't pray a lie: he wants Jim to be free. So where's the nightmare part? Huck is pretty convinced that helping Jim run away earns him an all-expenses paid ticket straight to Hell.
Escape? Yes. Return? Maybe not. Now that he's realized slavery is, you know, a corrupt moral system, Huck can't exactly return to his cozy pre-escapade life. There's no happy homecoming and settling down a changed man: once all the loose ends are wrapped up, Huck is off to "Injun" country to have new adventures.