The Gra identified idolatry inside the Jewish world

 Idolatry is equivalent  the Sitra Achra [the Dark Side.] This simple thesis I have thought for a long time but have not been able to bring much in the way of proof. But going by this simple identification much becomes clear as to why the Torah makes such a huge issue out of it.   And this has more practical consequence nowadays than people realize. For many of the mitzvot have a purpose to keep one away from idolatry and to in fact do everything just the opposite of what idolaters do.
There are numerous instantiations of this in the "reasons for the statutes" portion of the Guide. These all involve turning some idol custom around to its opposite. For example, the idolaters are clean-shaven, the Jewish men are to wear beards (Guide III, 37); the idolaters practice hybrid aboriculture, the Jews are forbidden to graft certain trees onto one another (ibid. III, 37); the idolaters pray to the morning sun in the east, the Jews are to set the Holy of Holies in the west and pray in that direction (ibid. III, 45); the tolemistic animals of the idolaters become the sacrificial animals of the Mosaic covenant (ibid. III, 46).
So while idolatry per se  nowadays can be hard to identify, the Dark Side is a lot easier.

The Gra helped a lot in this subject by identifying idolatry inside the Jewish world that was pretending to be keeping Torah. The verse in Proverbs also helps אוהב מוסר אוהב דעת ושונא תוכחת בער. People that hate Musar are from the Sitra Achra.
I should mention a few years ago there was a great edition of all the major works of Musar from the Publishing House Eshkol. They had all five books in one set. חובות לבבות, מסילת ישרים, שערי תשובה אורחות צדיקים ספר היראה המיוחס לרבינו תם. [I managed to get through them in my spare time and that added  a lot to character I think.]