Manley Pointer evidently has a fetish for objects related to disability, specifically for parts which have been tooled to replace what has been lost. He has a compunction to collect such objects from those who have become dependent on them. He tells Hulga as much:
I've gotten a lot of interesting things, he said. One time I got a woman's glass eye this way.
Manley sees Hulga's leg as a piece he can add to his collection, and he seems to have deliberately targeted her when he noticed her disability. It appears that his primary motive was to get hold of her leg all along. Also, it seems as if he resents Hulga's belief that she is more intelligent than him and that she looks down on him.
Furthermore, it becomes evident that he also wants to corrupt Hulga because he takes out whiskey, playing cards with dirty illustrations on them, and a blue box that might contain condoms. The fact that he has them hidden in a Bible indicates the depth of his perversion. It is ironic that he wants to corrupt Hulga because she had the same idea earlier; she had been thinking about seducing him. He assumes complete control once he removes her leg and places it out of her reach.
Manley perceives Hulga's question about him being "good country people" as an insult and states that her belief about him does not make her any better than him.
But it ain't held me back none. I'm as good as you any day in the week.
Manley believes that he is better than Hulga because he has easily misled her. He becomes angry that she dare insinuate that he is a Christian. He sees the act of believing in religion as nonsense. He venomously declares:
I may sell Bibles but I know which end is up and I wasn't born yesterday and I know where I'm going!"
He eventually tells Hulga that she is not that smart and that he has not believed in anything since he was born. When he leaves Hulga helpless, it is apparent that he thinks that she deserves being humiliated because of her supercilious attitude towards him. It becomes clear that his manipulation gives him a sense of power and achievement.
In Flannery O'Connor's "Good Country People," Manly Pointer claims to be a Bible salesman who is interested in Hulga. He flirts with her and the two of them go walking together the day after he comes to sell her mother a Bible. When they are up in the barn together, he convinces her to take off her prosthetic leg so he can see it. She does and he steals it from her. In doing so, he humiliates her. He does this for a couple of reasons. For one, it is a perversion he has to steal things like that from people--he admits as much and tells her all of the odd things he has taken from other people. The other reason he humiliates her is because he feels she acts superior to him with her higher education and imperious attitude. He enjoys taking her down a notch since he is a simple person and feels put down by her.