What event begins to change Hale's opinions about the arrests? How does he feel about the court?
In Act Three, Reverend Hale's opinion of Salem's court begins to change. For the first two acts of the play, Reverend Hale supports the court's decisions and rulings. In Act Three, Deputy Governor Danforth and Judge Hathorne reveal their corrupt, selfish nature by arresting anyone who seems to have opinions that challenge the court's authority. Hale initially witnesses how Deputy Governor Danforth arrests Giles Corey for contempt of court after he refuses to disclose the name of the person who overheard Thomas Putnam speaking to his daughter about accusing George Jacobs in order to buy his land. Hale also disagrees with Danforth's refusal to allow him to represent John Proctor, who claims to have evidence that the girls are lying.
Hale then watches as Proctor admits his infidelity with Abigail and Elizabeth lies about her husband's affair in an attempt to save his reputation. Before Danforth arrests John Proctor, Hale says,
Excellency, it is a natural lie to tell; I beg you, stop now before another is condemned! I may shut my conscience to it no more - private vengeance is working through this testimony! From the beginning this man has struck me true. By my oath to Heaven, I believe him now, and I pray you call back his wife before we. (Miller 114)
Hale then watches as Abigail and the other girls act like Mary Warren's invisible spirit is attacking them. Proctor is then arrested and accused of witchcraft, which forces Reverend Hale to denounce the proceedings. By the end of Act Three, Reverend Hale believes the court is thoroughly corrupt and heavily influenced by Abigail's lies. He ends up quitting the court and attempts to persuade the accused citizens to lie about being involved in witchcraft in order to save their lives.
I would say that the event that starts to turn Hale off on the court and the arrests is the thing that happens at the end of Act III.
At that point, Mary Warren has started to denounce Abigail Williams and the other girls. She has said they are lying. When she does that, Abigail and the girls start screaming about the bird and how Mary is bewitching them.
At this point, Hale gets disgusted. He stops having any faith in what the court is doing. He will continue showing this attitude in the next act.