In a Glass Darkly

Previously published in the print anthology The Regatta Mystery and Other Stories.
The narrator is startled by a vision in his mirror: He sees a man with a scarred neck strangling a beautiful blonde. He later meets the woman in his vision, Sylvia, and notes her fiancé's scarred neck. He tells her of his premonition, and the engagement is broken off. But is that all there is to it?