Guilty churchmen were ‘corrupted by Satan’
I believe the answer, simplistic though it may seem to some, is that these men allowed Satan to corrupt their very selves by neglecting humble and sincere prayerful contact with God, a vital part of their priestly training.
Christ gave the example of frequent prayer to God as vital in the continuous battle with Satan. He warned his future first Pope, Simon Peter, that Satan was the dangerous enemy who would strive to destroy him and the church. “Simon, Simon, behold Satan demanded to have you that he may sift you like wheat, but I have prayed for you that your faith may not fail, and when you have turned again strengthen your brothers.”