Macomber's beautiful wife, whom he married because of her beauty, secretly despises Macomber because she knows that he married her for one reason only: She is his "trophy wife." She despises herself because she knows that she married him for one reason only: He is very rich. He will never divorce her because he values her beauty; she will never divorce him because she has become comfortable with being a very rich wife.
Therefore, Margot is delighted when Macomber proves to be such a weakling and runs from the lion; it gives her psychological control over him. It's something that she can goad him with. However, when Macomber is about to reclaim his manhood as he faces the water buffalo, she is so frightened of losing control over him that she fires (or perhaps pretends to fire) at the charging water buffalo — and, instead, shoots her husband.