"And the incorruptible Professor walked, too, averting his eyes from the odious multitude of mankind. He had no future. He disdained it. He was a force. His thoughts caressed the images of ruin and destruction. He walked frail, insignificant, shabby, miserable—and terrible in the simplicity of his idea calling madness and despair to the regeneration of the world. Nobody looked at him. He passed on unsuspected and deadly, like a pest in the street full of men." (13.56).
The Secret Agent doesn't end with a comment on Verloc, Winnie, or even Stevie, but with a conversation between Ossipon and the Professor, two of the most unlikeable characters in this entire book. By ending on this note, Conrad isn't exactly showing us his optimism. Instead, he's suggesting that when the dust clears, it's cockroaches like Ossipon and nutjobs like the Professor who will be still standing.
In their final conversation, Ossipon feels overwhelmed by the "madness and despair" of Winnie's death. He also feels pretty guilty about betraying her and leaving her to die (we hope). When he asks the Professor about madness and despair, though, the Professor says that these feelings aren't part of modern society, because there is no passion in modern society at all—just boredom. Aww, Conrad. You should have lived to see the era of Advice Animals: where there be cute seal babies, there be no boredom.
In the final paragraph of the novel, the Professor leaves the Silenus Restaurant and finds himself back in the streets of London. He cant bear to look at how many people are walking around him, because he knows that there's nothing he can do as an individual that will make an impact on the world.
It's been only a week since Verloc's bomb went off in Greenwich Park, and already no one cares. But the Professor won't let go of his fantasies of greatness so easily. He continues to think that in a world of lazy, bored people, he is "a force." But then the narrator undercuts him, saying that he's "frail, insignificant, shabby, miserable." It's almost as if in this final passage, the Professor is struggling to think he's awesome even as the narrator keeps whispering in his ear that he's pathetic.
Ultimately, the Professor might actually believe in something bigger than himself. He might truly think that the world needs to be born again, and that this can only happen through destruction. But one last time, Conrad undercuts him, calling him a "pest in the street full of men." In the end, the Professor might survive. But, as Conrad asks us, what is the man surviving for?
After spending hundreds of pages talking about how awful the world is, Conrad can't offer us any clear ideas of how the world can change. The Professor says it needs to be completely rebuilt, but the guy's a lunatic. Ultimately, Conrad shows us that the world of capitalism is not a terribly good idea. But he doesn't have any alternatives, and he ends the book on a note of despair—maybe even madness.