In order to answer this question, it is important to see what Kino was like when he found the pearl.
When Kino found the pearl, he was filled with thoughts of grandeur. Greed and desire captivated his heart. Here is a quote:
In the pearl he saw Coyotito sitting at a little desk in a school, just as Kino had once seen it through an open door. And Coyotito was dressed in a jacket, and he had on a white collar and a broad silken tie. Moreover, Coyotito was writing on a big piece of paper. Kino looked at his neighbors fiercely. “My son will go to school,” he said, and the neighbors were hushed.
As the story progressed, he realized that people were envious of him. He also realized that people wanted to take advantage of him. For this reason, he became bitter and angry. He was not going to let anyone do this to him and his family. When Juana saw these changes, she tried to persuade him to get rid of the pearl, but he was resistant and at one point even struck her.
At the end of the novella, Kino's resentment turned into fear. He realized that people would kill him and his family to get what they wanted. Hence, we see a complete transformation in Kino—desire, resentment, and fear.