It is possible to have a protagonist in a story who never appears in person but whose motivation and influence is are felt throughout. A good example of this, with which Hemingway must have been familiar, is to be found in Henry James's greatest novel, The Ambassadors (1903). Mrs. Newsome, the rich and domineering mother of Chad Newsome, is the protagonist although she remains in America while everything take place in Europe. It is she who sends the middle-aged hero Lambert Strether to Paris to find her son Chad and persuade him to come home to take his place managing the family manufacturing business. It is Mrs. Newsome's strong motivation that drives the story. When Strether does not seem to be making much progress with Chad, who is having an affair with a married woman, Mrs. Newsome sends two more "ambassadors" to France, her stiff, prudish daughter and her daughter's husband. Since there should be an antagonist as well as a protagonist, it is clear that the antagonist is the cultured, sophisticated Mme. de Vionnet, Chad's mistress. The two women on two different continents are fighting over possession of the handsome playboy Chad Newsomee.
In the case of Hemingway's "The Killers," the protagonist is an unnamed "friend" who is paying the two hit men to kill Ole Andreson. The friend has been hunting Ole for quite a while, judging from what Ole tells Nick when Nick comes to the rooming house to warn him. So the antagonist in this story can be none other than Ole Andreson himself. The hit men would not be there if the friend was not paying them, and Ole would not be on the lam if he hadn't done something pretty serious to antagonize that friend. George, Nick, Sam, Max, Al. and Mrs. Bell are all just little people who happen to get involved in a battle between the protagonist and antagonist.
A protagonist is the character whose motivation makes the story happen. He is not necessarily the hero or the viewpoint character. I believe that the protagonist in Hemingway's "The Killers" never actually appears. He is "the friend" who has hired the two hit men to go to the town of Summit and kill Ole Andreson. We feel his presence even though we never see him or learn his identity. I would call Nick Adams the viewpoint character, although some of the action is seen through the eyes of George while Nick is bound and gagged offstage in the kitchen. The antagonist would have to be Ole Andreson, since he is the man the "friend" who presumably lives in Chicago wants to have killed. Ole has been on the lam for some time, trying to escape the vengeance of this powerful and seemingly omniscient man, undoubtedly a mobster like Al Capone.
Nick Adams is considered to be the protagonist in "The Killers". He is a young man sitting in the diner when the two hit men enter. He is still innocent regarding the reality of the criminal world of the 1920s. The men who have been hired to kill Ole introduce Nick to this world where violence and death are a regular part of doing business. Nick feels he must warn Ole of the danger, but Ole has resigned himself to accept the situation and just wait in his room for the men to kill him. Nick's innocence has come to an end, bringing him into the adult world. His young age leads him to express his desire to get away from Summitt because he sees no other way to deal with the situation.