In To Kill a Mockingbird, why does Atticus call Mrs. Dubose a model of real courage as opposed to “a man with a gun in his hand”?

Mrs. Dubose is a very difficult woman.  She seems to be unkind and generally grumpy.  The children, Jem, Scout, and Dill, are afraid of her.  Not only is she misanthropic, but she is very unappealing physically.  When Jem "decapitates" her flowers, Atticus punishes him by making him read aloud to Mrs. Dubose.  She doesn't seem interested in the stories, but he reads to her a bit more each day.  The children realize only later that Jem's reading was allowing her to pass the time, a bit more each day, while she kicked her morphine habit.  In the end, she dies free of the addiction and not beholden to anyone or anything.  This, Atticus says, is a sign of real courage. 

It turned out that Mrs. Dubose was not the person she seemed to be.  This is also true of Boo Radley and other characters in the novel and speaks to the idea that you never really know someone until you've walked in their shoes- an important theme in the novel and what Atticus says to Scout early on about the Cunninghams.