explain paps expectation and motivation

Pap shows up after years of being absent. He is a drunk and a deadbeat. He has heard of his son's good fortune, specifically about the reward money. Pap expects to show up into town and be a daddy one more. He is, of course, only interested in Huck's reward money.

To add to what Aslan has said . . . Pap wants Huck to work for him. He wants a servant, like rich folks have. He treats Huck worse than most whites treat their Negro slaves.