Use reasons and events from "Animal Farm" to prove why Old Major's teachings are wrong.
Use the Thesis Statement: "The Character of Old Major in the story Animal Farm denies truths about the nature of evil when he says all men are the enemy and claims that mans' removal will abolish the root cause of suffering."
The point it seems you are going for here would suggest that Old Major is espousing tribalism and discrimination in his vision for the future for the animals.
This seems like a reasonable criticism of Old Major's politics. Perhaps this is the reason that the revolution was doomed to fail - it was based, not on truly integral and righteous principles, but on an ultimately petty view of the world where a black-and-white division exists between animals and men; between good and bad. This kind of view is too simple and too selective in its engagement with reality to seed a political movement that will provide lasting benefit for individuals and societies.
This is a complicated question because it deals with the nature of evil more than the nature of man. The two basic opinions about man are that we are basically good and will do the right thing given the opportunity or that we are basically evil and will do the wrong thing given the opportunity. The myth of the ring in "The Republic" might help here. (Very short summary:) A shepherd finds a ring that can make him invisible if he wears it a certain way. Question: will his behavior when he is invisible be any different than when he is visible? Is our morality a cover for our fear of being caught, or is moral behavior part of our nature? The other part of the argument is whether evil is simply the absence of good or whether it is a real "thing."
All that for my comment on your question. If man is basically evil, the the Old Major (Marx) is incorrect. Changing the oppressor (in this case man) will finally make no difference because man's replacement (pigs) bring the same tendency toward evil that the former oppressor (man, pig or whatever) has. This seems to be the position that Orwell takes in "Animal Farm."
Political changes will have no lasting effects until we change the hearts of those who are ruling (man).
Napoleon/Jones ... same thing, different package.
Old Major is a political idealist. In chapter one he says that all animals are good and all men are bad and animals suffer because they are controlled by men. This is a grossly over-simplified statement and very soon proved wrong after the revolution when Napolean starts his reign of terror. Not all animals are good, far from it.
By generalising about 'all' animals and 'all' men, he is refusing to accept the complexities of real life, because that would destroy his theory. But he is willing to attempt to impose his theory on real life despite it being obvoius that it contains faults and is therefore unlikely to work.
He dreams of building a utopia for animals, but in reality, his simplistic ideology creates a dystopia.