Why does Atticus say that Jem's behavior is inexcusable?

Mrs. Dubose is an elderly lady who lives on the same street as the Finch family.  Whenever Jem and Scout walk by, she calls out rude comments to them.  Atticus has taught his children to be polite and respectful, so they do not respond to her in rudeness.

One day, Mrs. Dubose calls out especially rude things.  She uses a racial slur to describe their father's defense of Tom Robinson.  Jem becomes enraged.  He marches...

Mrs. Dubose is an elderly lady who lives on the same street as the Finch family.  Whenever Jem and Scout walk by, she calls out rude comments to them.  Atticus has taught his children to be polite and respectful, so they do not respond to her in rudeness.

One day, Mrs. Dubose calls out especially rude things.  She uses a racial slur to describe their father's defense of Tom Robinson.  Jem becomes enraged.  He marches off, and then grabs Scout's baton.  He goes back to Mrs. Dubose's house and hacks off the tops of her beloved camellia bushes with the baton.  When Atticus gets home from work, he confronts Jem:

Two geological ages later, we heard the soles of Atticus’s shoes scrape the front steps.  The screen door slammed, there was a pause—Atticus was at the hat rack in the hall—and we heard him call, Jem!"  His voice was like the winter wind.

Atticus switched on the ceiling light in the livingroom and found us there, frozen still.  He carried my baton in one hand; its filthy yellow tassel trailed on the rug.  He held out his other hand; it contained fat camellia buds (To Kill a Mockingbird, Chapter 11).

Atticus asks his son if he did it, and Jem confesses.  He tells his father why he is angry.  He understands his son's anger, but tells him that "to do something like this to a sick old lady is inexcusable."  Atticus suggests that Jem apologize to Mrs. Dubose, which he does.  Jem tells his father that he is truly not sorry, and Atticus is disappointed in him.