I would want to argue that the biggest moral lesson that can be extracted from this excellent novel would be the danger of greed. This is of course primarily shown in the way that the pearl transforms Kino throughout the novel as he seeks to improve his own life through selling the pearl.
Consider how Kino is presented at the beginning of the novel. He is shown to be a contented father, enjoying good relationships with his wife and son. However, the possession of the pearl changes all of this, corruping his innocence and contentment and transforms him into someone who is willing to kill others brutally to safeguard his pearl. The greed that dominates Kino is shown to dehumanise him, making him more like an animal than a human, leading him to beat his wife and also indirectly leading to his son's death. Note how he responds to his wife when she tries to get rid of the pearl:
Kino looked down at her and his teeth were bared. He hissed at her like a snake, and Juana stared at him with wide unfrightened eyes, like a sheep before the butcher.
Kino is described as being more like an animal, with his teeth "bared" and the simile comparing him to a snake. Even though it is his wife in front of him, Juana is described as being "like a sheep before the butcher." Greed has transformed Kino to such an extent that he would even kill his wife to preserve the pearl, and is shown to be losing his humanity.
The overwhelming message of this excellent story is that greed corrupts and destroys, and if we let it have dominion over our lives, as Kino did, we risk losing our very selves.