In Act One, Hale arrives as a self-proclaimed expert on the Devil and all of his wiles. He bears books, which, in his words, are "weighted with authority" and information regarding all of the guises of the devil and his minions. It is clear he feels he has significant experience in this whole area of dealing with the devil and his works. The part of the text you are referring to comes when he begins his efforts to discern what is going on with Betty Parris and Parris, scared, asks why the Devil should choose his house to strike and not any other house with less godly people in it. Hale responds as follows:
What victory would the Devil have to win a soul already bad? It is the best the Devil wants, and who is better than the minister?
Thus we see the thinking and logic of Hale. If the Devil is going to choose this village to attack, he is not going to bother working in a household with people whose soul is already bad - he will want to attack "the best" from God's perspective and thus it is entirely fitting that he would go for the minister's household in his quest for domination.
In addition to believing that the Devil might attack Reverend Parris's house because it would be no real victory to corrupt someone who is already corrupted, Reverend Hale seems to worry that the Devil would choose to attack Parris's house because the people living there may have invited him in. First, Hale interviews Abigail about what kind of dancing she and Betty were doing in the forest, then what was in the kettle on the fire; next, he asks Parris if there was anything living that went into the pot. Finally, he asks Abigail, directly, "Did you call the Devil last night?" Abigail, in a panic, tells him that Tituba, Parris's Barbadian slave, called the Devil and forced her to drink blood. Hale then asks Tituba if she is "gathering souls for the Devil." Therefore, in addition to his belief that the Devil likes a challenge, Hale is also concerned about the Devil in Parris's house because his daughter, niece, and slave seemed to have called him in.