question

The writer did it. While the boy approached the French windows, he sneaked around the back of the house, got in and axed the original murderer. Then during the few seconds the boy stood there frozen in shock, he quickly got out again, hit himself in the head with a rock and lie down on the grass, and when the boy found him he just pretended he had had been attacked from behind. It seems too imaginative and fantastic, but it is the only way he could have killed the other guy. But then, as he himself says to the police inspector: "When you have eliminated all the impossible..."

That bothered me too. Major suspension of disbelief. I don't see how it was possible to get from the inside of the house and get back to the backyard plus hit his head with a stone. What a disappointing ending.