How Awful About Allan

Allan leaves paint cans and thinner next to a heater one night, which causes a fire that kills his father and disfigures his sister, Katherine. He goes hysterically blind. After an eight-month stay in the mental hospital, Allan begins to come to terms with his resentful feelings toward his father and sister, and with the guilt that caused his blindness. Now he can see blurry images. He takes his doctor's suggestion that he talk about his feelings into a tape recorder. He tells the machine about his sister's mysterious new boarder, Harold Dennis, a university student who only speaks in a whisper due to a throat injury. He tells the machine how ambivalent he is about Olive, the woman to whom he was practically engaged before the accident. His finds his resentment over his sister's near-incestuous relationship with their father has not gone away, especially after learning of her work raising money to get the great professor's name on a new wing of the library. Allan learns that Katherine's boyfriend, Eric, had gone to Australia just after the fire, supposedly to make his fortune. Allan wonders if her disfigurement was the main reason he had left.Olive tries to repair her relationship with Allan, but her insistence that he join her in a ride downtown goes badly. He's waiting for her in the car when he gets the idea that a group of students are laughing at him. And then the roomer comes up and whispers to him menacingly. Despite his near-total blindness, he gets behind the wheel and drives off. He soon crashes into a street lamp, but without much damage to himself or the machine. But it's clear he's not well enough to be out in public.Things grow worse for Allan. He's sure the roomer is out to murder him. And who is this roomer anyway? Allan can't really see him. Is he really this Harold Dennis, or is he Eric, the boyfriend supposedly in Australia? His nerves grow worse. When a delivery boy comes to the door, Allan is so frightened that he accidentally slices his hand with a kitchen knife. Later, his fears drive him out into a thunderstorm, where he faints and lays for hours before someone finds him.A doctor making a house call tells Katherine that Allan should go back to the mental hospital. Meanwhile, Allan is dreaming about his childhood, especially about his cruel father and mocking sister.Allan manages to avoid the hospital for the time being. But now he's sure his life is in danger. He calls a cab to get himself out of the house, but the driver turns out to be Eric. He even whispers like the roomer and claims he has a cold.He's still in the house and terrified. But Olive tries to allay his fears. She tells him that the roomer can't be trying to kill him because ... there is no roomer. Olive says that according to Katherine there is no roomer and never has been one. Allan has been imagining his existence. But Allan knows better. The roomer's whispering is on his reel-to-reel tape recorder.In a way, Katherine's lie is no lie. There is no roomer. Katherine has been impersonating him, trying to drive Allan mad. And kill him. She traps him in the pantry and starts a fire. Allan manages to escape and learn the horrible truth about his sister. Even her disfigurement is a lie. Allan rips off the prosthetic patch over her face and finds that her scar is gone. She had had it removed, but wore the patch to make him feel guilty. It turns out his sister hated him for murdering their father. ("Murder" is her word.)Later, we find Allan and Olive happy together. Allan has recovered his eyesight. He receives a letter from his sister. She tells him there really was a Harold Dennis and he had really rented the room. But he called later to explain that his mother had died and that he wouldn't be needing the room after all. That's when she conceived the idea of impersonating him to drive Allan mad. In her letter, she asks for Allan's help and says that owing to his generous nature he's sure to give it. She wants him to speak to her doctors on her behalf and encourage them to release her. She has been incarcerated in the same hospital he had once been in.

Question: What caused Allan's blindness?
Answer: His guilt.