Who's Afraid of Virginia Woolf?

The film centers on the volatile marriage of a middle-aged married couple: George, an associate history professor at a small New England college, and Martha, the daughter of the dean. After they return home drunk from a party, Martha reveals she has invited a young married couple, whom she'd met at the party, for a drink. The guests arrive â Nick, a biology professor (whom Martha mistakenly believes to be a math professor), and his wife, Honey. As the four drink, Martha and George engage in scathing verbal abuse in front of Nick and Honey. The younger couple is first embarrassed and later enmeshed.
The wives briefly separate from the husbands, and upon their return, Honey reveals that Martha has told her about her and George's son, adding that she understands that the following day (Sunday) will mark his sixteenth birthday. George is visibly angry that Martha has divulged this information.
Martha taunts George aggressively and he retaliates with his usual passive aggression. Martha tells an embarrassing story about how she humiliated him in front of her father. Martha's taunts continue, and George reacts violently by breaking a bottle. Nick and Honey become increasingly unsettled, and Honey, who has had too much to drink, runs to the bathroom to vomit.
Martha goes to the kitchen to make coffee, and George and Nick go outside. The younger man confesses he was attracted to Honey more for her family's money than passion, and married her only because he mistakenly believed she was pregnant. George describes his own marriage as one of never-ending accommodation and adjustment, then admits he considers Nick a threat. George also tells a story about a boy he grew up with who had accidentally killed his mother and years later, his father, and ended up living out his days in a mental hospital. Nick admits he aims to charm and sleep his way to the top, and jokes that Martha would be a good place to start.
When their guests propose leaving, George insists on driving them home. They approach a roadhouse, and Honey suggests they stop to dance. While Honey and George watch, Nick suggestively dances with Martha, who continues to mock and criticize George. George unplugs the jukebox and announces the game is over. In response, Martha alludes to the fact he may have murdered his parents like the protagonist in his unpublished, non-fiction novel, prompting George to attack Martha until Nick pulls him away from her. George tells the group about a second novel he allegedly has written about a young couple from the Midwest, a good-looking teacher and his timid wife, who marry because of her hysterical pregnancy and money, then settle in a small college town. An embarrassed Honey realizes Nick indiscreetly told George about their past and runs from the room. Nick promises revenge on George, and then runs after Honey.
In the parking lot, George tells his wife he cannot stand the way she constantly humiliates him, and she tauntingly accuses him of having married her for just that reason. Their rage erupts into a declaration of "total war". Martha drives off, retrieving Nick and Honey, leaving George to make his way back home on foot. When he arrives home, he discovers the car crashed on the drive and Honey half conscious on the back seat and sees Martha and Nick together through the bedroom window. Through Honey's drunken babbling, George begins to suspect that her pregnancy was in fact real, and that she secretly had an abortion. He then devises a plan to get back at Martha.
When Martha accuses Nick of being sexually inadequate, he blames his impotence on all the liquor he has consumed. George then appears holding snapdragons, which he throws at Martha and Nick in another game. He mentions his and Martha's son, prompting her to reminisce about his birth and childhood and how he was nearly destroyed by his father. George accuses Martha of engaging in destructive and abusive behavior with the boy, who frequently ran away to escape her attention. George then announces he has received a telegram with bad newsâtheir son has been killed in a car accident.
As Martha begs George not to "kill" their son, Nick suddenly realizes the truth: Martha and George had never been able to have children, and filled the void with an imaginary son. By declaring their son dead, accordingly, George has "killed" him. George explains that their one mutually-agreed-upon rule was to never mention the "existence" of their son to anyone else, and that he "killed" him because Martha broke that rule by mentioning him to Honey.
The young couple departs quietly, and George and Martha are left alone as the day begins to break outside. George starts singing the song "Who's Afraid of Virginia Woolf?", and Martha responds, "I am, George, I am."

Question: Who killed the son?
Answer: George