he could do to take care of the rich people who lived in the stone and plaster houses of the town?

What is most dehumanizing about this doctor is that when he does find out about Kino owning the pearl, he immediately refers to Kino as “a client of his.” He even goes as far as rolling his eyes to visualize himself in Paris sipping wine at a restaurant. He also does not care when Coyotito is bitten by a scorpion—he won't treat him. Hence, greed makes a dehumanized doctor even more unethical, dehumanized, and cruel.

The second instance occurs when greed causes Kino to kill for the pearl. Kino is attacked, and in order to defend himself, he ends up killing the attacker. Kino realizes that protecting the pearl is now overriding his humanity.

He was an animal now, for hiding, for attacking, and he lived only to preserve himself and his family. He was not conscious of the pain in his head.

Kino, a family man who lived for his wife and son, is now a fugitive, a custodian of a rare jewel that people are willing to kill for. What kind of life can anyone live under those circumstances? How can anyone feel like a normal human being while being persecuted by the greed of the world?

Finally, greed takes over the humanity of the villagers. Everyone, from the priest to the beggar, no longer sees Kino as a fellow community member: he is the man who owns the pearl. He is an opportunity to become rich, rather than a fellow human being. All of them will go at any length to secure some benefit from Kino.

All of these are good reasons to support the thesis that greed dehumanizes people.

There are various thesis statements to be made about the allegorical message of The Pearl. You might say it is an allegory of the corruption that stems from wealth, or perhaps an illustration of the impossibility of predicting the future.

The latter thesis is particularly well supported by multiple points from the text. The major point, of course, is that Kino thinks that finding the pearl will mean great advancement and enrichment for his family, when in fact it brings only misery. A subsidiary point here is that the apex of Kino's ambition is for Coyotito, who will go to school, but in fact, Coyotito is the one who suffers the worst and dies.

Another point is that the scorpion, which brings the song of evil into Kino's family life, turns out actually to have no material effect on the story. Coyotito simply recovers from the bite, with no assistance except Juana's. A further related point is that, while Kino and Juana seek the doctor's help, this turns out to be the worst thing they could have done, for he is not only incompetent and mercenary but malevolent. Coyotito recovers without the doctor, but when news of the pearl reaches him, he comes to treat Coyotito and puts him in danger again, along with the whole family.

Your essay will be stronger if you write your thesis yourself, but here are some ideas: 

You may want to write a thesis that states that Steinbeck's 'The Pearl" is a parable (or fable, if you wish) of how individuals relate to the world. Your three points could show the main ways in which Kino's relation to the world changes. 

Of course, his finding of the Pearl of the World alters the attitudes of others, but his attitude also changes greatly.  For instance, when it is suggested that he be rid of the pearl, Kino declares, "The pearl has become my soul...If I give it up I shall lose my soul."  However, the greater evil seems to be his greed and holding onto the pearl without offering to share any of the wealth as well as his becoming a target of hit men. 

Check out the questions and answers on this book as well as the discussion group; two good links are listed below that may help.