Chapter 10 and 11: to kill a mocking bird

In chapter ten, feel bad for Jem, he loves football but his dad had a hard time playing because he was old (50) and he was busy. Also, this is the first time the title came up. Why is it a sin to kill a mockingbird? It has to be more than because they sing and don't ruin gardens. What's the big deal about this dog too- what's mad dog? Is that like a disease? I thought Atticus didn't like guns. He shot that dog very well though so I'm guessing something must have happened that made him dislike guns. 

In chapter eleven, Mrs.Dubose makes me very angry. She is very rude and obnoxious with her comments- to young kids who are about 8 and 10. I very much dislike her. Her thoughts are biased and racist. Why would she even say to scout and Jem, their father and the person he is defending are trash? Keep those thoughts to yourself. Why did she want Jem to read to her?