One major theme of the novel is the idea that evil cannot be defeated by one person alone. Juana and Kino represent the good and the members of the town, including the doctor and the pearl buyers, represent evil. Evil is allowed to thrive in the town because the evil men work together. When Kino faces them, he faces them alone. Because Kino chose to fight evil by himself, he loses. Had he been able to gather support from his village, he might have been able to ward off the men trying to take the pearl away from him. But Kino and his people don't seem to be able to work together to reach a goal. The only time they come together is to watch what happens to Kino. Some of them could have gone with Kino to the capitol, but Kino chooses to go by himself, with just his wife and baby by his side. He ends up losing his son and throwing the pearl away because of his inability to fight evil by himself.
One of the central ideas in Steinbeck's The Pearl is that wealth corrupts.
There is a related thematic undercurrent in the work that suggests that a lust for wealth is unnatural and can lead to a loss of perspective and/or an erosion of morals. Those who serve or pursue wealth in The Pearl are almost universally shown to be inhumane, willing to lie, cheat or kill for material gain.
Kino, the protagonist, is only one example of this ethos at work in the book, but he is also the person most affected by wealth and therefore most exemplary of this theme. Finding the pearl leads Kino to entertain new thoughts about himself, his life and his future.
"The pearl permits new and formerly impossible dreams, causing a dissatisfaction with the status quo of which Steinbeck approves" (eNotes).
At the outset of the narrative, Kino and his family are poor, but happy. After finding the great pearl and thereby becoming rich, Kino's life is ruined. People try to kill him, he is forced to kill, his house is burned down, he becomes changed and his son is killed. The wealth represented by the pearl determines each of these narrative elements.
"Every man suddenly became related to Kino's pearl, and Kino's pearl went into the dreams, the speculations, the schemes, the plans, the futures, the wishes, the needs, the lusts, the hungers, of everyone, and only one person stood in the way and that was Kino, so that he became curiously every man's enemy."
Thus, wealth fails to improve Kino's life and that of his family. Wealth, in fact, ruins his life. Causing him to kill several people, the pearl puts Kino on the defensive even as it creates a new set of potentially positive ambitions in him. The negative effects of new-found wealth clearly outweigh the positive effects both in regards to the events of the story surrounding Kino's family and in regards to Kino's character.
Other figures in the novel further demonstrate the ways that wealth can corrupt a person's character. Those who have the most wealth are also furthest from any sense of humanity or generosity. We see this in the doctor who refuses to treat a dangerously ill infant.
A desire for wealth can lead to a corruption of morals. When Kino was poor, he was happy and did not want for anything (except better health care). When he was rich, for a time, Kino's life and his character were utterly changed -- and not for the better.