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 is said by Proctor. He says this to illustrate that nothing in Salem has changed except that now the courts are believing the false accusations of people who want to get rid of other people, out of resentment or revenge. The accusers always end up being right.

In this case, Abigail wants to get rid of Elizabeth because she fired her, also taking Proctor away. Vengeance is writing the law in Salem.