Symbolism, Imagery, Allegory
Anybody Got a Match?
Black and white, right and left, darkness and light—all classic pairs that are used to describe good and evil in a slightly fancier and prettier literary way.
Throughout Mourning Becomes Electra, it's safe to say that if something isn't right, it happens at night, in the dark or behind closed doors where nobody can see. Christine murdering Ezra; Lavinia and Orin conspiring to murder Brant; Christine's and Orin's suicides—all go on after the sun sets. It's also safe to say that if it happens during the day, it's about making things okay. In other words, the day is usually when you see the friendly, polite and pretty side of things, when it's all about keeping up appearances, like during Ezra's funeral.
For our money, though, there's no better use of light v. dark imagery than when Orin goes off on Lavinia when Lavinia criticizes him for keeping the shutters closed:
ORIN: We've renounced the day, in which normal people live—or rather it has renounced us. Perpetual night—darkness of death in life—the fitting habitat for guilt! (The Haunted, Act 2)
Right here we see plain and simple that darkness is associated with death, guilt, and all things evil, while light is for normal, law-abiding, non-murdering folks.