I've seen this film at least 6 or 7 times over the past 20 years, but I never got it until tonight:
The dead 16-year-old son of Martha and George, used as a psychological tool between them, is George himself at 16. And the events in George's autobiographical novel about a 16-year-old boy "accidentally" shooting his mother with a shotgun and "accidentally" killing his father in a car accident and being put in an asylum afterward, suppressed by Martha's powerful university father for career reasons, really happened to George when he was 16.
Knowing this is the saddest fucking thing. And the utter lack of illusion between George and Martha one of the most deeply profound. What will they do once George has left the guilty 16-year-old boy behind, killed him off?
(Edward Albee, the author, is gay. We've all seen too much horror, both outer and inner, far too clearly.)