The initial impression I get from the incident with the rattlesnake is that Jim has now really gone up in Ántonia’s estimation. Previously, she’s acted superior towards him on account of their age difference. But after seeing how swiftly Jim has dispatched the rattlesnake, she’s actually rather impressed by him.
The incident also shows us a whole different side of Jim; this is an important stage in the development of his character. He’s always been such a dreamer, so deep and introspective, that it’s no wonder the more worldly and practical Ántonia never took him all that seriously. But Jim is at that delicate age where he wants to prove that he’s no longer just a little kid, and killing the rattlesnake gives him just such an opportunity. Though Ántonia still won’t regard him as anything more than a little brother figure, Jim has emerged from his duel with the rattlesnake as a more rounded character, more substantial and multi-faceted.
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.