Prospero uses Caliban as a slave, substituting him for the dukedom he lost.
Prospero has no respect for Caliban. He treats him as something otherworldly and less than human, using magic and the force of his will to subjugate him. This is partly because he says that Caliban tried to take advantage of Miranda, and partly because he wants to regain the power he lost when his brother conspired against him to eject him from his kingdom.
Prospero was Duke of Milan, until his brother Antonio and Alonso changed all of that. He was then cast onto the sea with nothing but a little boat and his daughter. He had some food and his magic books. It must be hard when you once were powerful, and you find yourself stuck on a little island with no one to make feel inferior. It is a good thing that he found Caliban to boss around!
In a conversation with the sprite Ariel, whom Prospero also keeps basically enslaved, he describes Caliban dismissively.
Dull thing, I say so: he, that Caliban
Whom now I keep in service. (Act 1, Scene 2)
Prospero is one of those people that just assumes that everything is his for the taking. Once upon this island, he decides that Caliban and Ariel should do his bidding. He is their superior in magic, so they should serve him. He describes to Ariel how Caliban’s mother was a “freckled whelp, hag-born—not honoured with/A human shape” and how he found Ariel trapped and screaming. He reminds him of how he let him out, so that he can remind him that he can always put him back in.
Prospero complains that Caliban is not a good slave because he does not give a “kind answer” (Act 1, Scene 2). He definitely rubs in the “slave” bit. He could definitely treat Caliban and Ariel better. Though he treats Ariel a bit better than he treats Caliban, Ariel knows how to simper a bit more and is magical and a bit more useful.
Caliban does threaten to curse Prospero often enough, and some worse things, but let’s face it, you reap what you sow.
Thou poisonous slave, got by the devil himself
Upon thy wicked dam, come forth!
As wicked dew as e'er my mother brushed
With raven's feather from unwholesome fen
Drop on you both! A south-west blow on ye(385)
And blister you all o'er! (Act 1, Scene 2)
An exchange like this results from the negative interactions that he constantly has with Caliban. If he had treated him better, from the beginning, instead of insisting on treating him like a slave and being so cruel to him, Caliban wouldn’t behave so nasty in retaliation.
In Ariel, Prospero can practice his magic and, especially when the shipwrecked royals arrive, his revenge. In Caliban he is enacting a fantasy where he is the duke and Caliban is his subject. He has his own little kingdom on the island, with his princess, and he can boss Caliban around at will. There is nothing Caliban can do about it because Prospero has complete power.
In Prospero's fantasy kingdom, there is no room for sympathy for Caliban. He is a slave, and a villain, and he deserves to suffer. In the end, Prospero forgives Antonio and Alonso, and lets Caliban and Ariel go. There is nothing left for him on the island. He is going back to his kingdom.
[to Caliban] Go, sirrah, to my cell.
Take with you your companions. As you look
To have my pardon, trim it handsomely.(Act 5, Scene 1)
This is the most polite thing Prospero says to Caliban during the entire play, and it is after he renounces his magic. Caliban, by the way, says he will be "wise hereafter,/And seek for grace." He regrets the things he has done wrong, including trying to kill Prospero by getting mixed up with clowns Stephano and Trinculo.
In the end, this is a play about forgiveness, not greed. It could have ended with so much bloodshed or destruction. Prospero had all of his enemies at his will. He chose to show them mercy. This is one of the last plays that Shakespeare wrote, and I think there is a lesson for us in that. In the end, it is about the choices we make.