Why is Scout surprised that Jem turns scarlet with anger when Mrs. Dubose defames Atticus for defending Tom Robinson in To Kill a Mockingbird?

Scout was surprised at how angry Jem became at Mrs.
Dubose's racial comments. When she made a snide reference to his father defending Tom
Robinson, accusing him of "lawing for niggers," Jem turned scarlet with anger. Jem's
reaction surprised Scout. She had never seen Jem so upset. He usually remained calm by
nature:

I took
it for granted that he kept his temper--he had a naturally tranquil disposition and a
slow fuse.

Indeed, Scout was
surprised that Jem lost his temper. Normally, Scout was the one protecting Atticus' good
name. She was usually in a fist fight when someone claimed that Atticus "was no better
than the niggers and trash he works for." (Mrs. Dubose's very comments).  Now, it was
Jem who became furious. He was so angry until Scout thought he had lost all his
senses:

At the
time, however, I thought the only explanation for what he did was that for a few minutes
he simply went mad.

Of
course, Scout is referring to Jem tearing down Mrs. Dubose's flowers. Jem seemed to be
crazy with rage. Scout reminded him that Mrs. Dubose has a pistol, but Jem continued to
beat down her camellias:

readability="9">

Jem did not begin to calm down until he had cut
the tops off of every camellia bush Mrs. Dubose owned, until the ground was littered
with green buds and leaves. He bent my baton against his knee, snapped it in two and
threw it down.

No doubt,
Scout stood in amazement at Jem's anger. She was shocked at what he did to Mrs. Dubose's
flowers. His actions were so aggressive and violent until Scout did not recognize him.
He was out of control with anger. Scout was stunned.