As a general comment on the injustices in Salem, the colony in Massachusetts was set up by Puritans who sought freedom to practice their religion without interference. However, when they set up their towns and villages, they did so with a sense of intolerance for all human feeling and anything that they did not understand. For example, when cattle die, or an infant dies in his crib, or a child is stricken with a strange illness, the people blame it on the forces of the devil among them or witchcraft.
The injustice exists because some individuals are blamed for unexplainable events as an easy way to dismiss how they happened, because a Puritan in good standing with God's will did not bring his wrath into the village or town. Rather than acknowledge that they didn't understand all the events that occurred, they found a way to explain the actions as the work of the devil, acting through someone in the town.
People accused of witchcraft were executed unless they confessed to being a witch, then they were spared. 19 people were executed in Salem, 400 were accused. Therefore, more confessed to witchcraft to save their lives and lived a life thereafter marked as a sign of evil on earth. Rejected by society, often losing her family's support and love.
In Act I- Abby shifts blame for Betty's state by accusing Tituba of using witchcraft to get the girls to go along with the events of the night in the woods. There would be severe repercussions for Abby and the girls being out in the woods and playing with spells in the Puritan society. Her fear for her own safety causes her to throw all blame on Tituba. Abby knows that she will bed believed over the black woman, and gets her out of trouble.
In Act II- Elizabeth is set up to be arrested to get her out of the way so that Abby will have John to herself. Abby plants a pin in a doll that she has Mary Warren present to Elizabeth. Later, she will feign a pain in the precise area of the doll's pin. Abby considers Elizabeth an obstacle in her quest to have John, and is willing to sacrifice her to get what she wants.
In Act III- Mary Warren attempts to set the record straight in court by telling the truth about the girl's accusations, but is prevented when the girls turn against her. She is not strong enough to stand up to them, and recants. The courts refusal to hear any reason causes the hysteria to continue.
In Act IV- Proctor is persuaded to lie and confess to spare his own life. The court is supposed to stand for truth, but by lying, the accused are not put to death. In the end, Proctor refuses to be a part of the injustice, and chooses to die instead.