The full quote from Macbeth:
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more: it is a tale
Told by an idiot, full of sound and fury,
The crux of the meaning of the para is that Life is not an organised story but instead (as John Lennon said) is what happens to you while you are busy making other plans.
An idiot's tale, therefore is a description that doesn't have an organised narrative. Things happen, and other things happen, but there likely is not a link between the events. The 'conclusion' is not obviously supported by the 'introduction' and 'body' of the story.
The point Proctor is trying to make is that the Holocaust is not a single story of an organisation (the Nazis) trying to exterminate other races. Along with all the reprehensible conduct displayed while the Nazis were in power, there were events that can only be described as beneficial to humanity. This is not in alignment with the general view that everything the Nazis did was evil, by definition.