White=evil? Black=good?

Twain portrays the whites as more evil than good in the story, especially Pap. Pap is a drunk abusive father. He got so drunk that he tried to kill Huck thinking he was the angel of death. He thinks that any money that his son has is automatically his. He controls and basically cages Huck in the cabin. Huck isn't allowed to do anything unless his father is there. What kind of father is that? Its one that doesn't deserve a son. He's an evil man.
Twain portrays Jim as a decent black man. He's extremely superstitious and not very bright. He is kind hearted though. Never wants to do anyone any harm. He's basically the opposite of Pap. What was Twain trying to accomplish by portraying Jim and Pap so differently? Why did he portray the white father as evil and the black slave as good?