I assume you are referring to Beatty’s lecture when he visits Montag at home. What I find interesting about Beatty in this scene and in the work as a whole is that while he is in charge of the destruction of books, he is also very well read.
I believe Beatty knows and understands what Montag is going through before Montag does. Beatty is essentially intimidating Montag into following the norm. He doesn’t outright accuse Montag of stealing books, but alludes to it.
Beatty’s message in the lecture is that books cause confusion and conflict in society and in man’s personal life. Books offer no answers only questions; therefore, they are dangerous.
I am not sure which point in the book you are talking about, but I think that his general way of trying to confuse Montag is to talk to him about how books really harm society. He tells Montag that books lead to the kinds of confusion that makes people unhappy.
Beatty's solution to this is to keep on being the way that Montag used to be. He wants Montag to just buy into the society's values. He says that doing this will give Montag the certainty and emotional stability that he wants.