In the end, it is discovered that Saul has been hurt by somebody that he had trusted. Who is this, and why is it so devastating?

We find out that Saul has been sexually abused by Father Leboutilier. This is especially upsetting because we think that Father Leboutilier was the one white person in Saul's life that protected him and looked out for his best interests. While Father Leboutilier did help Saul, this help came at a terrible price.