But that is only one factor in the sense of abstraction. Essentially, so the poem argues, we do not imagine the reality of terror. It would not be terror if it were reducible to everyday terms. It is the fear we can understand. This is an interesting point, though not especially poetic.
Life is contingent, whereas music, natural phenomena and murder by design are absolutes.
Terror may be seen as a perversion of the sublime. The fascist is often a failed artist, though it is not the failure of his art which produces the terror. Art and terror share a transcendence of the world's contingency. For this reason Plato found poets suspect, a notorious view which Iris Murdoch respects. The imagery of angels 'instant and imageless' underlines the sense of abstraction.