Humans are not truly evil. It is easier for Old Major, in encouraging the animals to dream of rebelling and taking over the farm for themselves, to paint humans as all evil. This sets up an "us against them" dichotomy, where the animals are cast as all good and the humans as wholly bad. While not true, it unifies the animals.
The chief human we learn details about is Farmer Jones. It appears he treated the animals decently until he fell into alcoholism. He is not truly evil, simply suffering from a disease (alcoholism) that causes him to neglect the animals.
Animal Farm challenges simple notions of one group being all good and another all evil. It shows that like humans, animals can be a mix of good and evil. The novel chiefly argues that it is too much power that corrupts both humans and animals. The pigs become like humans only because they have amassed too much power.