We learn that he is going to cover up the murder of Bob Ewell. He is doing this to protect Boo. He doesn't want this reclusive and odd man exposed to the entire community in a trial. So he explains to Atticus, who initially thinks Jem is responsible for Ewell's death, that Mr. Ewell just fell on his knife. He even goes so far as to demonstrate what happened using a knife he took of a drunk man earlier (this is Mr. Ewell who was drunk when he attacked the kids). While he wasn't able to do much for an innocent man, Tom Robinson (or maybe he wasn't willing to do too much for him - though Atticus explains earlier to Jem how Heck Tate really was on Tom's side), he is willing to cover for a guilty man, Boo. Remember, Boo might be odd and reclusive, but he is white. Is that the main reason why he covers up the murder?