What does Jem ask Walter Cunningham about at lunch? Why does Atticus say it’s a sin to kill a mockingbird?

Jem asked, “Your daddy Mr. Walter Cunningham from Old Sarum?”, and Walter nodded and Jem grinned at him and invited him to have dinner with them, assuring him that they would be glad to have him over, and he said: Jem said, “Our daddy’s a friend of your daddy’s. Scout here, she’s crazy—she won’t fight you anymore.”