The first thing you need to do is make sure you read the story carefully and pay particular attention to all the parts which include Old Misery. Old Misery is a very interesting character in the story. He seems to be a real mix of traits, and of course Trevor in particular takes advantage of his kindness to launch his plan.
The first thing we find out about Old Misery is a bit about his background:
Old Misery - whose real name was Thomas - had once been a builder and decorator. He lived alone in the crippled house, doing for himself...
So he lives alone, and his attitude with the boys shows that he is lonely and wants company. When Trevor asks to be shown around the house, Old Misery gladly gives him a tour, and is open about his trip.
Note too how Old Misery reacts when he discovers the boys have been using his toilet:
"I'm not unreasonable. Been a boy myself. As long as things are done regular. I don't mind you playing around the place. Saturday mornings. Sometimes I like company. Only its got to be regular.... And you come in at the front door and out at the back. No garden walls."
Old Misery therefore can be said to be a man who runs his life by traditional, ordered values, yet he is also lonely and desirous of company occasionally.
Of course, the overwhelming impression we are left with is one of pity by the end of the story, as the author closes his tale with an image of a sobbing Mr. Thomas grieving over the destruction of his house:
He gave a sobbing cry. "My house," he said, "Where's my house?"
"How dare you laugh," Mr. Thomas said. "It was my house. My house."
The laughter of the driver somehow exacerbates the pity we feel for Old Misery - his home has been completely destroyed and the landscape is now completely levelled around Wormsley - the boys have achieved their act of creation through destruction.