"Is this any more troubling than some high-profile school shooting that is likely to cause copycat shootings?"
While the age of the victims in a school shooting evokes sympathy, terrorism is more troubling because it is directed at the legitimacy of the state or society. If either unravels problems are significant and scale up.

To use school shootings as a test case, Columbine is a less threatening example of violence than was Beslan, because Beslan demonstrates how motivated terrorists could systemically strike at soft targets.