How did J. come to understand that Montmorency was not angelic and was actually quite suitable for life on earth?

Near the end of Chapter II, narrator J. tells us the story of how Montmorency came to live with him. The dog is a fox terrier, which is a fairly small breed. He evidently has an innocent-enough looking face, most of the time. This is what gives people the impression that he is angelic and somehow belongs to the heavens. “He will be snatched up to the bright skies in a chariot, that is what will happen to him,” J. reports people as saying.
Montmorency likes to chase cats, kill small animals, and fight with other dogs. Once J. takes ownership of him, he hears complaints from neighbors who claim Montmorency took part in 114 street fights, killed a dozen chickens, and trapped a man in his own tool-shed for several hours. J. had to pay the owner for the lost chickens, and he had to listen to a woman call him a murderer because the dog killed her cat. As we have already seen in Chapter I, J. tends to exaggerate for effect and humor. This is how he paints the first picture in our minds of what kind of dog Montmorency really is, so that we can use it for reference throughout the rest of the book.