Why does Roger murder the girls?

"Heat," by author Joyce Carol Oates, is a suspenseful and cautionary story about identical twins, Rhonda and Rhea Kunkel. The tragic tale of the Kunkel twins' murder is told through the retrospective lens of their mutual friend, who has since reached adulthood. The story opens on a hot summer day with the twins enjoying the fresh air outside, then switches to a tragic scene with the young girls placed in matching caskets. Roger Whipple is introduced at this point, but at first there is no hint as to his motive. It is only implied that Roger is the one who killed the twins.
In fact, keeping the reader guessing as to Roger's motivation is a significant literary device used throughout the story. It is revealed through the narrator's point of view that the twins were kind and bright, but they had no moral qualms about stealing from their grandmother. The narrator leads the audience to believe that this theft may have played a role in their untimely demises, but this is youthful oversimplification and an attempt to make sense of the senseless act of murder.
Roger is described as a 19-year-old with some undefined learning disabilities. He begins an inappropriate relationship with the twins, which culminates in their eventual sexual assault and brutal murder. Oates presents Roger as a metaphor for the harsh reality of menace and violence in the adult world, from which even children are not always safe. Roger represents all the destructive aspects of society, whether malicious or accidental, and brings about the end of the innocence set up in the early narrative. He is a stark contrast to the naïveté and mischievousness of the twins, shocking the narrator into a reality both as cold as the Whipples' ice house and as dark as the town becomes after the death of the Kunkel twins.
When the twins visit Roger's father's ice house in order to escape the heat of the hot summer day, Roger invites Rhea up to his room. Rhonda, lost and alone without Rhea, is afraid for the first time in her life. She tries to leave but the bond between her and her twin ultimately draws her back to the house and she goes to Roger's room to retrieve her sister. Although the narrative does not discuss the details of the murder or the events leading up to it, it is implied that Roger's motivation for killing the twins was related to the sexual assault. Whether he feared that they would tell what had happened or became angry because they resisted him, Roger's motives are left intentionally ambiguous.
Indeed, a central message of "Heat" is that terrible, tragic things happen to innocent people and, despite the narrator's best attempts to preserve her perspective of a just world, there is often nothing that can be done to prevent them and no justification for their occurrence. In this sense, the central message of "Heat" is that Roger had no genuine motive for the murder and that whatever motive he had is unimportant because the focus is on the senseless death of two innocent girls. The narrator eventually sees and acknowledges this as she notes, sadly, that she will always be scarred by the sight of the twins in their matching coffins and that their friends will always miss them.