Chapter 7 of the story My Antonia, by Willa Cather is meant to be quite allegorical. In a pastoral romance such as this, we tend to assume that all that happens will be prim, proper, innocent, natural....bucolic.
However, Willa Cather is seldom the type of author that would give you happy-go-lucky scenarios in her stories. As part of a naturalist/romantic story, reality plays an important role.
In the case of Jim and Antonia we find the quintessential replicas of Adam and Eve, sharing together a perfect paradise. Similarly, we find Antonia and Jim enjoying the beauty of the gardens, and nature. Yet, the snake lurks. Sin lurks. Problems do lurk even in the most amazing scenarios.
What Jim did at this point was to kill the lurking realities of life. It may have united Jim and Antonia in that she feels admiration for his actions. However, it also may help strengthen their relationship...or separate them forever.