What does jem ask walter cunningham about at lunch? why does atticus say it's a sin to kill a mockingbird?

These questions are from the novel To Kill a Mockingbird by Harper Lee.

1. What does Jem ask Walter cunningham about at lunch?

In Chapter 3, Jem found his little sister Scout rubbing Walter's nose in the dirt, at the schoolyard, and told her to stop and to let him go, alleging that she was bigger than he was. Scout refused to do so, but she soon gave up and things cooled down. Then, 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.”

2. Why does Atticus say it's a sin to kill a Mockingbird?

In Chapter 10, Atticus told Jem that if he were to shoot birds with his new air-riffle, he could shoot all the bluejays he wanted, but not the mockingbirds because it was a sin to kill a Mockingbird. Later on, Scout asked Miss Maudie, a close neighbor of the family, why Atticus had said that, to which she replied:

“Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.”