The most evident example at first glance is Farmer Jones. Old Major explains at the barn meeting why revolution is the only recourse possible. It is Man's nature to exploit, to take without giving anything in return.
The neighbouring farmers are also quite "human" in their roles, first bickering, then striking a compromise when it is in their mutual interest. The counterfeit money given in exchange for timber and their cheating at cards in the very last scene also exemplify the baseness of their nature.
Remember, though, that the animals themselves portray different aspects of human nature. Animal Farm is an allegory and a fable insomuch that the farm animals represent stereotypes as well as individuals. If Farmer Jones represents the last czar, Napoleon and Snowball represent Stalin and Trotsky. Bluebell's puppies, inoffensive at first, grow up to be Napoleon's secret police, a statement in itself of controlling the masses via repression and intimidation. The purges and executions which ensure echo real events in history. Hard-working Boxer and gentle Clover represent the dull-witted working class unable to react in time to protect its interests. The sheep and chickens are only too easily manipulated and brainwashed and offer no real resistance, either. The resolution and denouement of this tale is only too evident as wishful thinking and idealism prove to be no match against cold-blooded lust for power and ruthless tyranny.
The destructive nature of the humans is depicted in Animal Farm through their indifferent treatment of the animals, particularly when Farmer Jones forgets to feed the animals for a whole day due to his slovenly behavior and his drinking, and with regard to their attitude in slaughtering animals no longer able to work on the farm.
The catalyst of the revolt comes as a result of the animals barging into the store house to feed themselves, only to be caught by the farmer and his men who begin to whip the animals. The violence of the humans leads the animals to charge at them, spontaneously launching the revolution.
Also relevant to this topic is one of the animal's chief complaints against the humans that when an animal becomes too old to be useful on the farm, is no longer fit to work, immediately, the farmer, Mr. Jones and other farmers, takes that animal to the slaughter house. The animals in the story want to be able to retire to a field on the farm, once they are too old to work. This is particularly true for Boxer, the cart horse who works harder than any of the other animals.
The humans, in general in the story, are depicted as lazy, inefficient and apt to spend more time at the tavern, drinking than doing work on the farm. The animals make it very clear that man does not produce a single thing on the farm, unlike the animals who produce eggs, wool, milk and who work in the fields, man only takes from the farm, while contributing nothing.