what is jem and scouts attitude toward the father in the begining of the chapter

Atticus turns out to be not so "boring" as Scout and Jem first thought. They had felt that Atticus was not like other younger fathers who played football or baseball. Instead, Atticus preferred to read. When the rabid dog comes down the street, Atticus kills him with one shot and the children see a new side of their father.Miss. Maudie asks the surprised children,

"Didn't you know his nickname was Ol' One-Shot when he was a boy?"

What is the one thing atticus says is a sin to do while using their air rifles