Character Analysis
The Compassionate One
Stevie, Winnie Verlocs mentally disabled brother, keeps getting taken advantage of by all the jerks and posers of the modern world. There's probably no point that Conrad smashes us over the head with more than Stevie's compassionate nature. For example:
He could say nothing; for the tenderness to all pain and all misery, the desire to make the horse happy and the cabman happy, had reached the point of a bizarre longing to take them to bed with him. (8.88)
Stevie's a great guy, but his mind isn't all that practical. Sure, it'd be great to take everyone who's suffering and bring them into a big friendly (nonsexual) bed, but we know that if you ever tried this, you'd get arrested pretty quickly. Conrad knows this better than anyone. He shows mad respect for Stevie's niceness, but agrees that, at the end of the day, niceness won't get you very far in the real world.
The Violent One
When it comes to keeping other people from harm, though, Stevie isn't all sunshine and lollipops. He's actually a pretty violent dude. We can see this violent nature come out when he's setting off rockets in office stairwells, or when the narrator tells us that
At the bottom of his pockets [Stevie's] incapable, weak hands were clenched hard into a pair of angry fists. In the face of anything which affected directly or indirectly his morbid dread of pain, Stevie ended by turning vicious. (8.92)
It's actually Stevie's viciousness and violent tendencies that allows Verloc to take advantage of the guy. It's easy to forget this, but when Stevie goes to plant the bomb at the Greenwich Observatory, the dude totally knows what he's doing. He doesn't just think he's carrying some can of paint for Verloc. He's trying to blow something up. Again, its easy to think of Stevie as a simple, shining light of niceness in a jerky world, but Conrad's never going to give you that easy of an out. Stevie's nice, but when he hears about people suffering, he wants whoever is responsible to pay for it.
The Deepest One?
When he's not busy losing his mind about how terrible the world is, Stevie like to pass the time by sitting down at the Verloc's kitchen table and
… drawing circles, circles; innumerable circles, concentric, eccentric; a coruscating whirl of circles that by their tangled multitude of repeated curves, uniformity of form, and confusion of intersecting lines suggest[ing] a rendering of cosmic chaos, the symbolism of a mad art attempting the inconceivable. (3.17)
So what does the narrator mean by all of this fancy language? Well, if you check out the end of it, you see that Stevie's circles symbolize an "attempt at the inconceivable." This could basically symbolize Stevie's dream of making the world a perfect place, which Conrad tells us from the get-go is a totally impossible thing to do. Instead, the "cosmic chaos" part of this passage comes closest to the randomness and darkness of Conrad's world.
The circles that Stevie draws might also symbolize the circles that his mind is always travelling in. When characters like Winnie or Verloc come up against an impossible problem, they tend to shrug it off and figure it's not worth worrying about. Stevie, though, never stops trying to find new ways to figure out a problem. When he finds out the world's a rotten place, he thinks he should get the police to fix it, since he thinks the police are a "benevolent institution for the suppression of evil" (8.114). When Winnie tells him the police are corrupt, he doesn't know what to say at first, but keeps thinking about the problem. In a funny way, Stevie is actually a way deeper thinker than Verloc or Winnie.
But again, don't you go and try to make Stevie into the hero of this book. Conrad would never allow you such an uplifting moral stance. For all of his niceness, Stevie gets treated brutally and eventually killed by a world full of jerks. And this is basically the message Conrad attaches to him: its great if you're compassionate, but if you're not careful with your compassion, the world's going to destroy you. That's the modern world, says Conrad. He doesn't like it; he's just telling it like it is.