They pause at the edge of a cliff above the lower circles — the stench rising from below is so unbearable that they shelter behind the tomb of Pope Anastasius II (accused of heresy). While they wait for their senses to adjust, Virgil delivers a systematic lecture on Hell's moral structure. It is the most important expository passage in the Inferno: everything below is organized.
Hell is structured around Aristotle's Nicomachean Ethics. Sin falls into three broad categories: Incontinence (the failure of reason to control appetite — lust, gluttony, avarice, wrath, which occupy the upper circles); Violence (the malicious use of will against others, self, God, or nature — Circle VII); and Fraud (the corruption of the rational faculty itself — Circles VIII and IX). Fraud is worse than violence because humans share violence with the beasts, but the capacity to deceive through reason is specifically human, and its corruption is therefore a deeper betrayal of human nature. Simple fraud (Malebolge) wrongs strangers; treachery (Cocytus) wrongs those who trusted you — and is the deepest sin.
Virgil also addresses why usury is in Circle VII with violence: it offends Nature (which produces wealth through human labor and art) and Art (which follows Nature). The usurer extracts wealth from money itself, from the mere passage of time, without working — a violation of the cosmic order of productivity.