There is not good reason to eliminate this possibility, while some problems might be in fact from world views gone astray or mental illness. It has been the tendency of the West to minimize or eliminate entirely the effects of the Dark Side in peoples' lives and to deny its existence.
The Gra would not have put that group into excommunication if he if not think that the Sitra Achra had not become mixed up with it in some kind of hidden way.
This is an important topic and I would like to at least explain my own approach.
Mainly it goes like this: morality and holiness are tightly bonded. It is as simple as that.
Morality here means common sense morality. When you see a person that does not have that or a group, then they are part of the Sitra Achra.
Common sense morality is what you would think based on the Ten Commandments. Do not steal or lie etc. Once a group is involved in some kind of fraud I assume there is more wrong with it than moral wrong.I also assume there is metaphysical evil inside of it.