If this were true though, then man's behavior should be guided solely by his instinct, but they guide their lives according to their ideology. This contradicts their own theory. Communists talk about freedom and brotherhood all the time, but only in order to deceive people. Freedom, equality and fairness cannot exist in the Communist world. The only equality we can find there is the equality of animals, no more.
Instinct governs the animal world.
Animals don't think of others, they put themselves first. Consequently in a materialistic society everybody wants to be first. The Communist authorities know this, so they control everyone very strictly. They control everyone, because if the people are given freedom they will just kill each other, as their theory suggests. Just as we would confine a wild animal in a cage, they want to keep people in a cage.