Answer:
The light is mirrored in Mary's persona.
Step-by-step explanation:
Both lamps in the room are "alight" at the start of the short passage. The lights in the grocery store are still turned on. It's dark outside when the officers arrive to investigate, and Mary catches "the flash of a torch through a chink in the curtain."
The light is mirrored in Mary's persona. At first, she appears content and happy, as signified by the lamps being lit. Her spirit darkens after she murders her husband, and it darkens much more when she orders the officers to consume the murder weapon.