The original question had to be edited down. I think that the idea of injustice in both works can be seen in that the legal system fails to deliver justice. The legal system of justice in Salem does not deliver justice, primarily because it is controlled by individual interests. The controlling interests of those in Salem's positions of power are to guarantee convictions on the grounds of being a witch. This precludes a full sense of justice from appearing. In this, there is injustice evident. A similar reality can be seen in Forster's work. Justice is absent when Aziz's trial is infiltrated with so much of politics from both sides. There can be no real pursuit of justice in this setting. To a large extent, regardless of Adela's recantations, justice for Aziz was impossible because so many in the community had competing interests and sought to use the trial for advancing their own agenda. It is here in which both works suggest that legal justice is impossible, creating a state of injustice.
In a larger sense, I think that the weight of accusation helps to create a reality of injustice for people in the respective works. In Miller's rendering of Salem, the accusation of witchcraft carried with it an almost certain burden of guilt, unless individuals "named names" and thus created more injustice. This helps to enhance the feel of injustice throughout the entire drama. At the same time, life in Chandrapore carries with it injustice through accusation. This weight is something that Aziz must bear in full force. Yet, in her recanting and seeking to stop the machine that was in operation long before she got there, Adela also suffers the weight of injustice. She is rejected, for the most part, by the British community because she could not go through with what they so wanted. The weight of accusation brings about injustice to Adela as the narrative develops. In both works, injustice through accusation is another shared idea.