How you start your body paragraphs depends on your thesis statement, which controls your essay. If this is just one paragraph, then the first sentence of the paragraph is your topic sentence. In other words, a thesis statement controls an entire essay while a topic sentence begins and controls only a paragraph.
Depending on what you want to discuss about "The Sniper," that is what your body paragraph should introduce. If, for example, you want to talk about the sniper being able to think through how to get off the roof before dawn, when he would be vulnerable, or about how he has to use a trick to get rid of the other sniper, your first line in the body paragraph should introduce that idea. If you want to talk about the event of finding out that the man he has killed is his brother (the story ends with him looking in the dead sniper's face, so no emotional or psychological impact is even alluded to), you begin with that idea. If this is an essay and your body paragraph is the first one of three body paragraphs, you begin with the first idea introduced in the thesis statement.
So think of what you want to discuss, such as the futility of war, the impact of war on the families, the never ending vigilance required by war if you are a soldier, or the determination required of you if you want to be a sniper. These then become 6he content of your body paragraph or paragraphs.