Parris tells Danforth, "You cannot hang this sort." What does he mean?
By this point in the story it is obvious to Rev. Hale, and I believe to Parris, that these people are moral, they are telling the truth that they are not witches, and that they will go down as martyrs, not villians if hung at this point.
The reading audience knows the truth, it appears that the townspeople are able to see through the girls and their spectral evidence. The only ones still hanging on the to lies as if they are truth are the magistrates.
This play so interestingly demonstrates man's awareness of his own sin. John knew his, wrestled with God about it, and received forgiveness. Parris, a holy man, wanted to hide the truth from his flock and got worked up about money and church attendance. Repentant heart for these things? No, not really. So, you get the chance to determine the measure of a man in this piece. I think that is part of what Parris is realizing in these simple words. Proctor's life was worth preserving, he had much to teach his sons and was ready to live a righteous life, but admit mistakes.
When Parris says this, he is arguing that Danforth should not allow John Proctor and Rebecca Nurse to be hanged. What he means by this is that they are too well respected to be executed.
To see this, look about a page or so before the line that you are asking about. There, Parris says to Hathorne that Rebecca Nurse and John Proctor are people who the community cares about. He compares them to people who have already been hanged and says that they are not like those people.
The reason that Parris cares about this so much is that he is afraid. He is afraid that the people of the town will rise up against him the way that the people of Andover have done. So he wants Danforth to refrain from hanging respectable people like Nurse and Proctor.