In The Crucible, why does Hale say he has come "to do the devil's work"? What motivates his action?
Hale is motivated only by his quest for the truth. He wants to rid the town, the colony, of "the devil's work" so that he can better serve God.
When we are first introduced to Reverend Hale, he believes that everything he needs to help the town of Salem can be found in his religious books. Remember that the Puritans were very superstitious people, and attributed many events to supernatural causes. For example, if a crop died, it wasn't explained with with poor soil or drought or weather conditions, it was thought to be the will of God because the farmer had somehow displeased Him.
While Reverand Hale is an educated man, his faith blinds him to the evils within many of the characters of the play until it is almost too late. He sincerely believes that everyone is as honest and earnest as he is, forgetting that human nature often leads people to act in their own self interest at the expense of others.
His interrogation of Tituba in Act 1 is a prime example of this. He unknowingly leads Tituba to give him the "correct" answers so that she can avoid punishment (lashes with the whip). She eventually calls out the names of the townspeople in order to divert attention from herself, and her "testimony" is substantiated because a man of God, Rev. Hale, accepts what she says as the truth. Her accusations cause Abigale and the other girls to call out names of even more townspeople, and thus the hysteria begins.