“And for the sin which we committed against you unwillingly or willingly.” The sin is actually done unwillingly; however it is rooted in the will.

For every Jew was given the ability to conquer every impediment and obstacle to Torah and spiritual service, and only with the weakening of the will does the duress win. After a person is involved in an inner search during the days of selichot, Rosh Hashanah, the Ten Days of Repentance, and the eve of Yom Kippur, he reaches the understanding that the cause of the duress is the will.

(Igrot Kodesh Admor Harayatz, vol. 2, p. 430)