The truth is buried so far down in "The Purloined Letter" that you'd need a backhoe to unearth it. Everyone in the story weaves deceptions: the royal lady, D—, G— (his little trick with the muggers), and Dupin (the green glasses). Even our narrator isn't totally straightforward, because why else would he need to be so cryptic about everything? Is he withholding evidence—or is he just putting us in the position of the detectives, sifting through evidence to get to the truth in a world that doesn't much value it?
D— did not actually deceive anyone; he simply counted on the royal man's inability to notice what was obvious.
Because the royal lady's deceit sets the entire plot in motion, she is the story's most culpable villain.