What do the humans do that makes them evil in Old Major's eyes

From the text:

Because nearly the whole of the produce of our labour is stolen from us by human beings.

"Is it not crystal clear, then, comrades, that all the evils of this life of ours spring from the tyranny of human beings? Only get rid of Man, and the produce of our labour would be our own. Almost overnight we could become rich and free. What then must we do? Why, work night and day, body and soul, for the overthrow of the human race!