The original title for this book was Verloc, and over the course of 1906, it changed to The Agent. By the time he published the novel, Conrad had come up with The Secret Agent: A Simple Tale. The title is provocative because even though the book deals with very complex issues in an often confusing way, Conrad decides to call it "A Simple Tale." Is he making fun of us?
He might be (this book is pretty brutally snarky at times)…or he might be tying to say that no matter how complex things seem, the general message of the novel is quite simple: human beings are not nearly as good or important as they seem to think they are. Bummer, Conrad. Thanks to you, we'll be drinking cocoa and streaming Netflix and thinking about our insignificant lives all night. Jerk.
Also, by changing the title from one mans name (Verloc) to the name of a job (The Secret Agent), Conrad shows us that the problems of the world don't just come from Verloc alone, but from everyone else like Verloc and from the kind of corrupt society that rewards lazy people and liars (i.e. the "secret agents" of the world) with cushy lifestyles.