We know one is not supposed to worship an intermediate. And the Rambam considers that to be the main prohibition of idolatry. So it is easy nowadays to see the reason for the signature of the Gra on the letter of excommunication. I can understand that at the time of the herem/ excommunication it was not clear to most people what the problem was. So most people ignored the herem (excommunication) that the Gra signed. But\ nowadays it is abundantly clear.
[Even Rav Nahman mentions this important principle in the LeM vol I perek 62. One is not supposed to worship an intermediate. So one could ask on why Rav Nahman seems to contradict himself in terms of the need to a "tzadik." I am not sure how to answer this, but I still feel that Rav Nahman himself was not included in the herem, so I feel more or less at ease in learning what is possible from his writings.
