Why do they call him old Misery?
Old Misery – whose real name was Thomas – had once been a builder and decorator. He lived alone in the crippled house, doing for himself.
In the short story "The Destructors" Mr. Thomas is referred to as "Old Misery" by the neighborhood boys. An old man who lives alone in a crippling house and only emerges once a week to buy groceries. His house is one of the only in the neighborhood that survived the bombings during World War 2. He belives that that old ideas of respect for elders still exists, and while he doesn't care for the boys, he allows them to use of his outside bathroom (or loo).
The boys play a trick on him and his house and destroy if from the inside out while Old Misery is away for two days. T (whose real name is Trevor) comes up with the idea after being invited in one day by the unwitting Mr. Thomas.
The destruction of his house completes his journey into misery.