"Once more go back a little to the point,"
I said, "where you state usury offends
The divine goodness, and untie the knot."
"Philosophy, to one who understands,
Points out — and on more than one occasion —
How nature gathers her entire course
"From divine intellect and divine art.
And if you pore over your Physics closely,
You’ll find, not many pages from the start,
"That, when possible, your art follows nature
As a pupil does his master; in effect,
Your art is like the grandchild of our God.
"From art and nature, if you will recall
The opening of Genesis, man is meant
To earn his way and further humankind.
"But still the usurer takes another way:
He scorns nature and her follower, art,
Because he puts his hope in something else."
Dante puts the usurers in the lowest sub-circle of the seventh circle of hell, with others whose sins are regarded as doing violence against nature and nature's God; many people have noted that usurers are placed deeper into hell than violent murderers, violent suicides, blasphemers, and sodomites. Dante regards usurers as perverting art, i.e., productive skill, by means of which we are supposed to produce and create and thereby imitate the goodness of God. Usury is the anti-art: it produces nothing substantial, being just a set of multiplication games with money, and therefore does not really contribute anything to 'earning one's way and furthering humankind'. It merely gives the illusion of doing so, and is therefore a sort of mockery of both human reason and divine providence -- indeed, a sort of universal violence against neighbor, God, and one's own reason, an extraordinarily efficient form of violence by which you do the most damage with the least effort.
The Violent in Dante's hell suffer violent punishments; the punishment of the usurers is to sit while futilely and wearily trying to ward off with their hands the violent, whipping winds that sometimes shower them with burning sand and sometimes cover them with flaming fire.
(ADDED LATER: By an interesting coincidence, Arsen Darnay had a post on usury and Dante as well.)