Explain "... vengeance is walking in Salem!"
In Act Two, officials of the court travel to John Proctor's home to arrest his wife after Abigail claims Elizabeth sent her spirit to murder her. After Reverend Hale informs Elizabeth that Abigail has charged her with murder, John tears the warrant and says,
"I’ll tell you what’s walking Salem - vengeance is walking Salem" (Miller, 77).
John Proctor is commenting on the overwhelming vengeance throughout the community, which motivates accusers to claim their enemies are witches in order to enact revenge for certain matters. In the austere community of Salem, neighbors harbor animosity towards each other for various reasons, which are now being made public under the guise of witchcraft.
For example, the Putnam family resents the Nurse family for leading a faction that prevented one of their relatives, James Bayley, from being elected minister of Salem. As a result, the Putnams accuse Rebecca Nurse of being a witch and murdering their children. Abigail's accusation that Elizabeth tried to murder her is another example of vengeance throughout the community of Salem. Abigail resents Elizabeth for kicking her out of her home and having a relationship with John, the man she desires. Overall, private vengeance is becoming public as neighbors enact revenge on one another for past offenses by claiming that their enemies are involved in witchcraft.
"Vengeance is walking in Salem!" means that people are using the witchcraft accusations to get revenge on those who they believe have hurt them, or those against whom they feel jealous, or angry, or just negative in general.
It is important to know how Puritan societies behaved during this time. In general, people were supposed to be worshipping or reading the Bible if they were not working. Any behavior outside of that was considered to be a "vain enjoyment", which was definitely frowned upon. Keep in mind that many people considered it their duty to judge the behaviors of others, and it was considered patriotic and holy to point out the wrongdoings of others, and especially to accuse them of consorting with the devil.
Take that mood of the time, judging others, and add the fact that you can get back at anyone by accusing them of being a witch, and you are using those accusations as vengeance.
"Vengeance is walking in Salem" because one of the long-standing reasons that the citizens are accusing each other of witchcraft is due to grudges they have been holding for decades. In other words, it's not that the accused have actually done what they are accused of doing, but that someone is accusing that individual for his/her own deep-seeded reasons. Since there is such hysteria behind the witch trials, the accusations continue to fly wildly. Vengenace, in that way, has taken human form, and is moving freely through the town of Salem.