The original question had to be edited. I think that one reason why "Killings" is a more effective title than "Killers" is that it does not dehumanize. In the title, "Killers," the natural judgment is to perceive the people who murder as less than human. They are not human if they are "killers." The term, "Kilers," implies a moral judgment that only "bad people" are "killers." The focus of "Killers" is to place emphasis on those who are "killers," causing less reflection to the results of such actions.
Yet, in "Killings" a moral ambiguity seems to develop. In "Killings," the focus is the death that results and not necessarily a rush to judgment on who did it. Matt and Ruth are in anguish over the death of their son, and yet, they end up bearing responsibility for committing the same heinous act. They are good people, by all accounts. Yet, they do a horrific thing. In this, there is intricacy evident, and difficult to form a clear moral judgment. The end of the short story is one in which there is moral ambiguity and complexity. We do not seem to be left with "killers" as much as confronted with the only certainty of "killings." The tears that Matt feels when he thinks of Frank and Richard is significant. The death of both young people is the only moral certainty at the end of the story. It is in this where "Killings" becomes more significant of a title than "Killers." The conclusion of the story and the themes that come about as a result are ones in which death and "killings" are all that constitutes certainty. Everything else is shrouded in complexity and intricacy.