Tanya Chapter 24, Class 06 (186).

Tanya Chapter 24, Class 06 (186)
Considering that when doing an Aveira one is creating a Klipa lower than (the worst Hashem Himself created) Avoda Zara; when doing any Aveira one must be possessed by a Ruach Shtus (temporary insanity) or he would never do it.
More specifically, there are two levels to this Ruach Shtus: 1) allowing oneself to sin; 2) drawing distinction between one Aveira and another, ‘this I will never do’ (but this is ok to do.

Tanya Chapter 24, Class 06 (186).