Miller is arguing that people get carried away and lose their sense of reason and right and wrong.
In the play, a few young girls turn everyone’s lives upside down when they are caught dancing in the woods. Before you know it, their simple lie, designed to save themselves, has turned into hysteria. People actually believe there are witches among them, even though there is no actual evidence.
Proctor, who is caught up in the problem when his wife is accused by a girl he had an affair with, explains it best.
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 will not give my wife to vengeance! (Act 2)
Good people who have done nothing wrong are being accused of witchcraft. Sometimes it is just because someone needs a name, but often it is because someone has a grievance against someone. They use the court to get revenge on someone they feel has wronged them.
The fact that such horrible things can happen in the name of God is ironic. The people have grown so afraid of God and the Devil that they are willing to kill each other. However, the main reason it is allowed to happen is because they let it. They allow fanatics to guide them, such as Danforth.
Now, children, this is a court of law. The law, based upon the Bible, and the Bible writ by Almighty God, forbid the practice of witchcraft, and describe death as the penalty thereof. (Act 3)
Even while other towns come to their senses and disband their courts, Salem persists. It is caught up in a mass delusion, and the hysteria is self-perpetuating. The people are afraid, and out of fear they go along with it. People confess who are not guilty (of course no one is actually guilty), and they are caught in a trap. If you confess you are saying you are guilty, and if you don’t confess you are assumed to be guilty.
Arthur Miller was caught up in the mania of McCarthyism because he was accused of being a communist. In order to send a message, he later wrote this play to demonstrate the real danger of a witch hunt. He used an actual historical event: The Salem Witch Trials.