Tools of Characterization
As we discuss in "Symbols, Imagery, Allegory," appearances (and particularly eyes) are representative of what's going on internally for the characters in The Flies. If Clytemnestra's eyes look dead, it's because she's abandoned her freedom and self, and is no longer living as a human being. When Orestes experiences the epiphany resulting in his transformation, his physical appearance changes accordingly. (Electra notes the change in his eyes.) Notice, too, that the guilt consuming Electra in Act III manifests itself physically – she appears torn and ravaged in the span of a few hours.
Remember, for an existentialist, actions are all that really matter (as opposed to words, aspirations, or beliefs). Orestes's transformation in Act II isn't a real transformation until he acts in the second scene. Likewise, his commitment in Act III isn't legitimate until he acts on it by swinging open the doors of the temple and walking out into the sunlight. Orestes is an existential hero because his choices are made through actions and not through words.