Montag also mentions later that Beatty wanted to die. One other possible interpretation: Knowing Montag had books, Beatty seemed to give him the opportunity to return to his old life as the mindless fireman: sending the Hound was his last chance to do this. Being tipped off by Millie's friends (and then Millie herself), Beatty then proceeded to make Montag burn his own house with the other firemen just standing around watching. As Beatty shouted out all those quotes from authors, I kind of got the feeling that Beatty was trying to get Montag fired up (pun intended): fired up to the point where he would have the guts to become the rebel that he eventually did become. Maybe, in Beatty's twisted reverse-psychological (and suicidal) mind, maybe Beatty was also fed up and decided to go out in a blaze while indirectly giving some last minute inspiration to Montag.
Beatty tries to help Montag by giving him a pep talk about why firemen are so important and why books are so dangerous. By doing this, he is trying to get Montag back into a more "normal" frame of mind -- one that will not be a danger to Montag.
Beatty pretty much already knows that Montag has books. He could probably get Montag in a lot of trouble if he wanted to. But it seems that he really wants Montag to change his mind and renounce the idea of books. He tries to tell him what was so bad about books and get him to go back to being happy about burning them.