It had started the night before when he had wakened and heard the lion roaring somewhere up along the river. (2.10)
Macomber lets fear get the best of him. His act of cowardice is not a one-time thing. It was long coming and corrosive.
"What range will it be?"
"Can't tell. Lion has something to say about that. Won't shoot unless it's close enough so you can make sure."
"At under a hundred yards?" Macomber asked.
Wilson looked at him quickly. (2.23-26)
Poor Macomber. He tries to comfort himself by finding out how far he'll be from the lion when he shoots him. Wilson can already sense that something is wrong here, and check out his manly way of speaking. He doesn't even bother using an article in the first sentence, or a subject in the second. He's too busy thinking about the hunt.
"It's that damned roaring," she said. "It's been going on all night, you know."
"Why didn't you wake me, she said. I'd love to have heard it. (2.41-42)
Margot begins needling Macomber when she hears that he has been up all night scared. She subtly puts him down by suggesting that the lion's roar would have delighted her. All right, Margot, we're calling your bluff.