This phrase is contained in a speech found in Act II of The Crucible by Arthur Miller. It is spoken by John Proctor at the height of the witchcraft/accusation frenzy in the courts of Salem. At the center of the storm is Abigail Williams, who is manipulating the girls to continue a farce they began in a simple attempt to avoid being punished for some high jinx in the forest one night.
Proctor knows the truth and, now that his wife has been accused, he is willing to speak what he knows, even though it may cost him his life. He tries to convince the court that the girls are lying and others in town are taking advantage of the situation to benefit themselves, but no one wants to listen. Even the more level-headed Reverend Hale doubts Proctor's word and suggests that if Elizabeth Proctor is truly innocent, the courts will discover it. Proctor is furious, filled with righteous indignation because he knows the truth.
PROCTOR: If she is innocent! Why do you never wonder if Parris be innocent, or Abigail? Is the accuser always holy now? Were they born this morning as clean as God's fingers? I'll tell you what's walking Salem-vengeance is walking Salem. We are what we always were in Salem, but now the little crazy children are jangling the keys of the kingdom, and common vengeance writes the law! This warrant's vengeance! I'll not give my wife to vengeance!
He makes the case that nothing in Salem has changed except that now the courts are believing the false accusations of people who have something to gain from getting rid of people. The accusers are always right. In this case, Abigail wants to get rid of Elizabeth, and this is how she is paying Elizabeth back for firing her--and taking Proctor away from her. Parris and Putnam are also part of this trouble, all out of vengeance and spite.
In short, the law of the land is now driven by vengeance rather than either biblical principles or legal principles. Vengeance writes the law.
This quote comes from Act Two. Abigail has pushed a needle into her belly, and a needle has also been pushed into Mary Warren's "poppet." This doll was to be given to Elizabeth Proctor, who Abigail accuses of having used witchcraft and the doll to place the needle in her.
Proctor asks Hale how it is that Abigail is never accused of a crime. He then states in his frustration:
"We are what we always were in Salem, but now the little crazy children are jangling the keys of the kingdom, and common vengeance writes the law!"
He is saying that individuals like Abigail are using the law as a way to exert revenge on members of the town. The law no longer represents justice. Rather, Abigail is utilizing superstition and fear to take advantage of the legal system and punish those whom she chooses. In this case, it is Proctor, with whom she had an affair, by accusing his wife.