I do not think Rutherford was a Marxist. The idea of there being "classes" among God's people was already ingrained from the Russell era, and Rutherford just made up new classes of his own. It's almost impossible to think as a JW without being class conscious in this spiritual sense.
Russell had his own classes. The little flock, of course, was tops. But there was also the Great Company which was conceived of as a secondary heavenly class. It was a class which Russell thought the anointed fell into if they were not faithful enough to rule with Jesus in the kingdom, but weren't bad enough to go into the second death. This class would end up in an angelic existence below the little flock. (Together these composed the "church class" which Russell also called the "sanctuary class" of Daniel's prophecy.) The great mass of nominal Christians (which today are considered nothing but "Christendom"), because they at least had faith in Jesus as their Ransomer, were considered to be justified (declared righteous). But in his mature thought, Russell considered them to be only "tentatively justified" unless they came into the church class, at which time they would be fully justified.
Russell conceived of Armageddon as a social revolution, a conflict between capital and labor, but I recall one scholar (I don't remember who) felt that Russell had never directly studied Marx, but had simply been exposed to socialism generally. Of course, Russell had no intention of instigating a revolution - he simply predicted this would be God's way of cleansing the earth before the establishment of the new order. I think that this view became too much of an embarrassment once the revolution occurred in Russia, and that's why Rutherford's Armageddon is much more like the traditional end of the world (accomplished directly by divine agencies, but without destroying the planet).
But Russell's class thinking in his theology was not, I believe, derived from socialism as much as it was from generally observing the social classes of his day. I understand that Europeans are very much more conscious of class than Americans. I wonder, though, if the New England of Russell's time could have contained more class consciousness.
So Russell translated social class into spiritual class, and Rutherford took it from there because class was already imbedded in Bible Student/Jehovah's Witness theology.
Justin