Idioms – What is the Idiom or Metaphor for ‘National Fabric’

LET GOD SORT THEM OUT

The OP has stated that the phrase in question, "means that one considers all the instances of some group as equally bad ... "

The meaning of the OP's phrase is about not caring, as opposed to, not knowing. Additionally of great significance, the phrase is an expression of vituperation and disdain, conveying a fundamental lack of concern or moral consideration for any who fall within this category (choose your favorite 'scapegoat') because these have been judged to be the equivalent of "feces," and there are no "grades" of feces, each and all are merely and only, "shit."

While admittedly not an exact equivalent, there is a phrase which pithily sums up the same blithe lack of moral concern and invective, “Kill them all; let God sort them out." This phrase is considered to be a derivation of "Caedite eos. Novit enim Dominus qui sunt eius", a phrase allegedly spoken by the Papal legate and Cistercian abbot Arnaud Amalric, prior to the Massacre at Béziers, a massacre in the French town of Béziers that formed the first major military action of the Albigensian Crusade. A direct translation of the Latin phrase would be "Kill them. For the Lord knows those that are His own." See, Wikipedia Caedite eos. Novit enim Dominus qui sunt eius. The phrase was possibly a misunderstood reference to 2 Tim. 2:19 which, in part, reads, “The Lord knoweth them that are his”.

One idiomatic phrase that may be relevant is Job's comforter. William Benét, The Reader's Encyclopedia, second edition (1965), has this entry for the phrase:

Job's comforter. One who intends to sympathize with you in your grief, but says that you brought it on yourself, thus in reality adding weight to your sorrow.

The reference is to a central element in the Old Testament's Book of Job. Three friends—Eliphaz, Bildad, and Zophar—come to see Job after multiple calamities have befallen him (through no fault of his own); and they argue, consecutively, "If thou wert pure and upright; surely now he [God] would awake for thee" (Bildad), and "God exacteth of thee less than thine iniquity deserveth" (Zophar), and "For thine iniquity teacheth thy mouth ... Thine own mouth condemneth thee" (Eliphaz). Job then responds, "I have heard many such things: Miserable comforters are ye all." (All quotations are drawn from the King James translation of the Bible,)