Maybe Hemingway sensed that there was something lacking in his story because of the absence of women. That may have been part of his reason for adding a scene in which Nick Adams encounters Mrs. Bell, the assistant landlady, at Mrs. Hirsch's rooming house. Nick meets her when he rings the front doorbell, and she escorts him upstairs to Ole's room. Then after his conversation with Ole, Nick runs into her again on his way out. She seems to have been waiting for him. She tells him a great deal about Ole Andreson, including the fact that he has been in his room all day. The presence of Mrs. Bell in the story suggests that Hemingway did not deliberately want to leave women out. It was just that there was no place for a woman in the main scene in Henry's diner.
Hemingway was an admirer of Joseph Conrad. At the end of Heart of Darkness, Conrad has Marlow go to visit the fiancee of Kurtz, and Hemingway may have felt that some such meeting with a naive, sheltered woman would be appropriate for his own story, providing a contrast with the preceding horrors.
Interestingly, Hollywood, which has produced at least two films based in part upon "The Killers" have not only included women in the screenplays, but have given one of the female roles a pivotal part. For example, in the 1946 film noir starring Burt Lancaster as Ole Andreson in his screen debut, the beautiful Ava Gardner plays a major role. However, there is much more to the screenplay's plot than what Hemingway has written--and the femme fatale is always important to film noir.
Regarding Hemingway's short story, although Hemingway was a "man's man," his omission of a female character does not seem to be chauvinistic or patriarchal. Rather, a female character in the context of Hemingway's theme is irrelevant. For, "The Killers" has as its theme Maculinity and "the code" to which Hemingway's males must adhere. Thus, "The Killers" is as much the story of Nick Adams as it is of Ole Andreson, for in this narrative, it is Nick who is learning the "Hemingway code" of masculinity, of grace under pressure, exhibited by Andreson and Sam. For this reason, critics argue over who is the protagonist--Ole or Nick since Ole Andreson already possesses this ability to face death with dignity while Nick is the proselyte.