As the Blessed Virgin stood beneath the Cross, there was still another source of most bitter sorrow for her, namely, her clear conception of the nature of sin. We may not doubt that Our Lord imparted to His Mother a supernatural knowledge of sin, its wickedness, its hideousness, and the borrow and hatred that God entertains for it. The hideousness of sin she beheld written on the Cross in the Precious Blood of Jesus. She saw the sins of the whole world, like a mountain, pressing upon the bowed shoulders of her bleeding, dying Son. She saw that He was truly God, thus crushed by the numberless sins of man, covered with their guilt, crucified by their malice! What unspeakable suffering for the sinless, guiltless, Immaculate Heart of our sweet Mother!
Another source of Mary's unutterable grief was man's ingratitude for Our Lord's Passion, and the eternal perdition of so many souls for whom her Son's Precious Blood would be shed in vain. She, the Queen of the Apostles, the Mother of the Church, saw passing before her eyes a vision of the heedlessness of mankind for sins forgiven, the relapses into mortal sin, the amazing, frightful multitude of venial sins, the cold indifference, the disgust for Divine things, the desecration of the holy sacraments, the rejection and abuse of grace on the part of so many souls ---- all, consequences of the basest ingratitude.
Source: About Devotion to the Mother of Sorrows. Nihil Obstat: Gulielmus J. Blacet, J.C.L. Censor Librorum Imprimatur + Joannes P. Cody, S.T.D. Episcopus Kansanopolitana-Sti. Josephi March 10, 1951