Why is Pap angry with Huck in The Adventures of Huckleberry Finn?

In chapter 5 of The Adventures of Huckleberry Finn, Huck finds Pap, his drunken, abusive father, waiting for him in his bedroom. Huck now inhabits a separate world from Pap, since he lives with the Widow Douglas and is rich and respectable, having found a fortune in gold at the end of The Adventures of Tom Sawyer. He is dressed in fine clothes, and the Widow and her sister, Miss Watson, are attending to his education.

This change in Huck's status infuriates Pap, who accuses Huck of putting on airs and thinking himself superior to his poor, uneducated father. He tells his son angrily,

You've put on considerable many frills since I been away. I'll take you down a peg before I get done with you. 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? I'll take it out of you. Who told you you might meddle with such hifalut'n foolishness, hey?—who told you you could?

Pap is also angry that he cannot get his hands on Huck's money. In the previous chapter, Huck realized that Pap had returned, and he went straight to Judge Thatcher, who was holding his money for him and giving him the interest. Between them, Huck and the Judge arranged a legal ruse, giving the Judge not only control but ostensible ownership of Huck's fortune. This means that Huck can truthfully tell Pap that he has no money except the dollar he has on his person. Pap takes the dollar and gets drunk with it but is angry that he cannot get the rest of the money from Judge Thatcher.

Who is Pap and why is he an antagonist in The Adventures of Huckleberry Finn?

Although there are many antagonists in The Adventures of Huckleberry Finn, Pap is one of the first we are introduced to early in the novel.  We first see Pap when Huck finds him unexpectedly in his bedroom at the Widow Douglas's. Twain describes him as looking sickly gray with greasy, long hair that hangs in his face.  His clothes are in tatters--his toes stick out the end of his shoes, and the top of his hat is busted out.  Pap is an alcoholic who also symbolically represents Twain's idea of the low life poor who are racists, liars, and cheats (We are introduced to many throughout the novel).  He shows up to the Widow Douglas's house to claim the money Huck and Tom find in The Adventures of Tom Sawyer.  Huck is rich, and for the first time, Pap is interested in Huck's "well-being" because of the money he can get his hands on.  Pap is in conflict with Huck who does not want to give money to his father to gamble and drink away.  In reaction, Pap kidnaps Huck, takes him to a cabin in the woods, and locks him in the cabin when he goes to town to drink.  He is abusive to Huck and even tries to kill Huck one night when he comes home drunk.  It is then that Huck fakes his own death and escapes to Jackson Island to get away from Pap and society.  

We later learn that Pap was murdered when Jim confesses to Huck that he covered up Pap's body in the house floating down the river so Huck wouldn't see his Pap dead.  

Pap would probably win the "worst father award in literature" if a vote was taken.  He is a despicable character with no redeeming qualities.  In essence, Huck is the exact opposite of his father.