"The Man Who Read John Dickson Carr," by William Brittain

In this story, Edgar decides to emanate his idol, the mystery novelist John Dickson Carr, by committing a locked room murder that would be worthy of a mystery novel.

He would have pulled it off too, except for one very important detail: he forgot to lock the door. 

In Brittain's story we can see the use of means, motive, and opportunity as Edgar schemes to commit the perfect crime (one that he cannot be found guilty for). 

What is the only possible perfect crime?

Motive We know he has a motive to kill his uncle (excluding Edgar from his will), and so does his uncle's lawyer and doctor. 

Means We also know that he is a grown man who is physically able to kill his uncle.  He had knowledge of the chemical reaction to start the fire after he went up the chimney.

Opportunity So in order for Edgar to get away with murder he must cast doubt on his opportunity to commit the murder by creating a locked room scenario.  If he couldn't have gotten in or out of the room, how could he have killed his uncle?

All of his preparations where to prove that he did not have the opportunity to kill his uncle.  He cleaned the chimney, and wore all white clothes.  He hid the sword in the corner.  He got the chemical paper to light a fire after he escaped through the chimney. 

All this was so that the two witnesses could not possibly place him at the scene of the crime regardless of his obvious motive or means. 

Yet all of his efforts were futile, being that he forgot to lock the door.