the cult that the Gra signed the excommunication on

The problem I see with cults [Hindu, the cult that the Gra signed the  excommunication on, etc.] is that the leader can absorb people into his archetype.
Especially in the West. This is because the West has been emptied of meaning by David Hume and the British  Empiricists. That is if one holds by Nominalism and or empiricism the world has been emptied of meaning. This is what gives secular society the empty feelings that people feel but can not express. Thus youth are prey for archetypes that can absorb their energies and spit them out.

I can not go into all the details, here but you can easily on your own find examples of this type of thing and even see it in your own lives when you saw someone get involved in some cult.

Cults are the reincarnation of the ancient idols. Joining a cult one becomes a servant to some archetype. This at least gives me an idea of why the Gra saw fit to put the entire movement  into excommunication. Not just that he saw it as kelipat noga, but rather that he singled it out as the most destructive force coming into the world at the time that would result in other cults like communism and other assorted idolatries.