The awakened Nostromo perceives that those who argue for grand, selfless causes do so from selfish motives. It is quite true that Monygham's important position in town depends upon San Tomé. But this is a multi-layered line in which San Tomé also stands for the dream-ideal. Remember that it is Monygham's "ideal conception of his disgrace" that enables him to live and act; without the dream-ideal he would "be nothing" in a deeper sense. Continuing upward to the epic metaphor of this chapter, I believe one can read this line as saying that the Devil himself would "be nothing" were the dream-ideal to be annihilated -- because it is only through the dream-ideal that the most poignant human tragedies are acted out. Though on both levels the Monygham/Devil character is restoring society from a benevolent impulse, on both levels the underlying "interest" is open to question.