Racists believe that if African Americans are sufficiently provoked, they will retaliate with an orgy of indiscriminate violence against white people. This is why Roof went to Emmanuel AME; to kill black people in a particularly sacred spot. He had done his racist homework: how many 21 year old whites would have understood Emmanuel AME 's historic importance?
The racist faith is that a sufficient provocation would ignite massive black violence. It is their hope, because it would then
justify their own massive anti-black violence. That's what they are stockpiling all those guns for.
(That African Americans don't respond to these provocations as the racists expect seems to never sink in. Race War just does not seem to be the African American fantasy.)
The racists expect that eventually the federal government would intervene to protect African Americans. Their anti-black pogrom would morph into an insurrection against the US government. Some state governments, they expect, would join them. That's why they never let go of that confederate flag. They want to fight that war again.
Texas mobilizes its National Guard in response to a US Army training exercise. Texas brings its gold back to Texas. I doubt such moves are actual indications of secessionist planning in the Texas government, but they indicate the grip that these insurrectionist fantasies have on rightwing mind.
So, on the one hand, the attack on Mother Emmanuel was a terrorist act, in that it wanted to frighten, humiliate and intimidate African Americans generally. But it was also a terrorist act in that, like the attack on the World Trade Center, it desired to provoke a response, to start a war.