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Sure, Huck's father Pap may be an ignorant, abusive, alcoholic racist who beats his son and extorts whiskey money from him, but he's not all bad. He's got some really redeeming qualities—like…
Okay, we lied. He has no redeeming qualities whatsoever, and we don't really feel sorry that he removes himself from Huck's life entirely by dying. But at least it's easy to figure out Pap's motivations: he wants whiskey. And really, this is about all you need to know about Pap. He's an addict. He'll do anything to get more whiskey, including lying, stealing, and abusing his son. (As if you needed a cautionary tale about late-stage alcoholism.) He's so addicted to alcohol that he lies around drunk in the pigpen and has delirium tremens-induced fits of hallucinations.
When the new town judge tries to reform him, Pap is so un-reform-able that the judge changes his mind about the ultimate good of human nature and declares that there are some men you can only reform with a shotgun. Is Pap proof that no one can change? Or is he just a man in the grip of a terrible addiction?
The drinking is bad enough, but that's not even the worst of it. The worst is that Pap is a willfully ignorant racist. He doesn't want Huck to learn anything, saying "You've put on considerable many frills since I been away… You're educated, too, they say—can read and write. You think you're better'n your father, now, don't you, because he can't?" (5.6). In other words, he's jealous because his son knows more than he does. Some dad, right?
And it's not just his son. He can't handle the idea of black people knowing more than he does, either:
here was a free n***** there from Ohio—a mulatter, most as white as a white man. He had the whitest shirt on you ever see, too, and the shiniest hat; and there ain't a man in that town that's got as fine clothes as what he had; and he had a gold watch and chain, and a silver-headed cane—the awful- est old gray-headed nabob in the State. And what do you think? They said he was a p'fessor in a college, and could talk all kinds of languages, and knowed everything. And that ain't the wust. They said he could VOTE when he was at home. Well, that let me out. Thinks I, what is the country a-coming to? It was 'lection day, and I was just about to go and vote myself if I warn't too drunk to get there; but when they told me there was a State in this country where they'd let that n***** vote, I drawed out. I says I'll never vote agin. (6.11)
Pap is so outraged that a black person is (1) educated, (2) well-dressed, and (3) allowed to participate in the political process that he just refuses to vote.
And we say, good riddance.