It's a difficult choice, but Goya had a great fear of madness and his black paintings in the Quinta del Sordo are often seen as an expression of this. Insanity was a great preoccupation of his. I translated a (rather controversial) book about his black paintings a few years ago, and I think the theme of these is very much more madness than injustice. He of course equated injustice with madness, and they are closely tied in the word "sinrazón"!

In this context I think sin razon refers more to something unjust and non logical.

sinrazón.

1. f. Acción hecha contra justicia y fuera de lo razonable o debido.

I think it definitely refers to injustice. Could be subjective, but I prefer iniquity.
wordreference.com:

1 injustice, unfairness, iniquity
an unjust act
2 evil, immorality, wickedness, iniquity
morally objectionable behavior
3. iniquity, wickedness, darkness, dark
absence of moral or spiritual values; "the powers of darkness"

the irrationality of the system brings out the madness of injustice