Despite their friendship, Huck has always considered Tom more civilized and smarter than he is (even when events show otherwise). When Huck is impersonating Tom, not by choice, Tom is thrilled with the opportunity to play games and roles, but even more thrilled with the idea that they are going to steal Jim out of slavery. Huck, being of a more simple mindset, thinks that societal disapproval of stealing and especially stealing slaves is enough to keep any but the worst people from the act; Tom, on the other hand, sees it as the chance for an adventure and has no concern for morality or ethics.
"I know what you'll say. You'll say it's dirty, low-down business; but what if it is? I'm low down; and I'm a-going to steal him, and I want you keep mum and not let on. Will you?"
His eye lit up, and he says:
"I'll HELP you steal him!"
Well, I let go all holts then, like I was shot. It was the most astonishing speech I ever heard -- and I'm bound to say Tom Sawyer fell considerable in my estimation.
(Twain, The Adventures of Huckleberry Finn, gutenberg.org)
Huck is shocked that Tom would want to participate in such a low and foul act. Tom believes that he is allowed to do more-or-less anything in the pursuit of "adventure" as long as he is not directly hurting anyone; he also believes that adventures should be done according to "tradition," or stories and rumors. Huck is worried about societal disapproval, and also wants to just steal Jim and be done with it, without all the unnecessary complications that Tom insists must be added.