Tom Nook is an evil dictator racoon who rules over the innocent town of Crossing. He controls the entire economy, because all the money you get or give comes from and goes to him. He builds people houses then charges massive amounts of money for them, ensuring that people are always poor. He runs the only shop in the town that doesn't pay rent, and guess who all the rent payments go to. That's right, Nook. So, he runs a shop and all his competitors pay him to exist. Not only that, but he gives you money for random junk like shells and rocks and stuff, only for you to pay it back to him via mortgage payments, taxes, everything. So, essentially, Nook is the master of Monopoly.
Not only that, but he also oppresses people. At first he pretends to be all nice and friendly, but he drops even that guise as his bank acconut swells, wearing a suit and a tie and living in the most obscene luxary, employing his two nephews as slaves to run his shop. Resetti despises Nook and refuses to conform to his ideals, resulting in his banishment from the land. Nook really is a fascist dictator hell-bent on conquest. You can't leave the town without his permission, otherwise you cannot return. Not only that, but you can't even speak to him without him trying to sell you some overpriced doohicky that does nothing. And he builds you a house with a floor that 'gives out' (i.e, falls in) if you try and put too much on it.