This is a good question. Much of the tension and climax of the chapter is filled in darkness. Kino, Juana, and Coyotito are going to sell the pearl. So, they head north. However, they realize that they are being tracked by three men. In a very real sense, the family of three become animals and the trackers become hunters.
As they hide in the shadows, Kino has a plan to attack them, but he loses his chance as the moon comes up. At this time, the men hear a cry. They do not know if the cry is human. They, nevertheless, shoot and unbeknownst to them, they hit and kill Coyotito. It was a dark night. This is the setting and this is how the characters are feeling.
Kino is able to kill the men, but in the process he loses his son. Finally, he and Juana make their way back home to discard the pearl. But darkness still looms. Here is a quote that show this:
"...the sun was behind them and their long shadows stalked ahead, and they seemed to carry two towers of darkness with them."
In a sense, their best days are behind them. The setting is dark, they are hopeless, and this is how they feel.