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.