I have long thought that the prohibition in the Torah not to do idolatry refers to the Sitra Achra [the Dark Side. The realm of Satan.] So it is clear why the Gra would have signed the decree of excommunication.
But if one asks what obligation does anyone have to listen to the Gra? I would answer that this comes up in the laws of Nedarim and Shavuot. The achronim [authorities after the Beit Yoseph]] in fact discuss what force does any excommunication have? [We know it is valid because the laws concerning excommunication are brought in tractate Moed Katan] But why is it valid? The Achronim say it is derived from the law of a neder [(oath) נדר]. A person can forbid his object to another person by saying: "This object is a "karban" קרבן [an animal dedicated as a sacrifice] to you."
But my point is that the signature of the Gra is not what makes the Sitra Achra [the dark side] to be evil. The Satan is evil anyway. It is just the signature of the Gra that makes it more clear.
[Just to be a bit more clear. A neder [(oath)] is brought down in the baMidbar (The Book of Numbers). It is forbidding an object to oneself or to another. It is not the same thing as a shavua (to swear). Both are laws that come from the Torah. So when we have laws of "Cherem" (חרם) or "Nidui" (נידוי) these have the force of Torah laws. [But that is only when the court that makes the excommunication has sufficient authority to do so. Not anyone who decides to put another person into cherem can do so.] So when the Gra signed that letter of cherem, it has validity in so far as anyone who violates it is automatically in cherem themselves and that everyone is obligated to listen to it. [I.e., just like a person can forbid his object to another person--even if that other person does not like it or agree--so a cherem brings about an automatic obligation to listen to it even if you do not agree with the premises on which it is based.
But if one asks what obligation does anyone have to listen to the Gra? I would answer that this comes up in the laws of Nedarim and Shavuot. The achronim [authorities after the Beit Yoseph]] in fact discuss what force does any excommunication have? [We know it is valid because the laws concerning excommunication are brought in tractate Moed Katan] But why is it valid? The Achronim say it is derived from the law of a neder [(oath) נדר]. A person can forbid his object to another person by saying: "This object is a "karban" קרבן [an animal dedicated as a sacrifice] to you."
But my point is that the signature of the Gra is not what makes the Sitra Achra [the dark side] to be evil. The Satan is evil anyway. It is just the signature of the Gra that makes it more clear.
[Just to be a bit more clear. A neder [(oath)] is brought down in the baMidbar (The Book of Numbers). It is forbidding an object to oneself or to another. It is not the same thing as a shavua (to swear). Both are laws that come from the Torah. So when we have laws of "Cherem" (חרם) or "Nidui" (נידוי) these have the force of Torah laws. [But that is only when the court that makes the excommunication has sufficient authority to do so. Not anyone who decides to put another person into cherem can do so.] So when the Gra signed that letter of cherem, it has validity in so far as anyone who violates it is automatically in cherem themselves and that everyone is obligated to listen to it. [I.e., just like a person can forbid his object to another person--even if that other person does not like it or agree--so a cherem brings about an automatic obligation to listen to it even if you do not agree with the premises on which it is based.
