How does George Orwell portray that humans are destructive by nature in "Animal Farm"?

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.