Socialism is not really a form of goverment, its an ideology, marxism and onto communism are governmental forms. The construct of capitalism being the enemy of the proletariat was brought in by Marx as part of Marxism. Capitalism was portrayed as being the same system evolved from feudalism, with the same protagonists at the top oppressing the masses. Hence where the socialist vs capitalist dogma that has abound since.
PS many in the field do not regard the USSR as ever having practiced communism, rather more a direct form of dictatorship.