Just a quick thought: Terrorism is about attention more than anything. Violence is the tool, and fear is the fuel, but in the end it’s about attention. On the micro level, I have never seen strangers so enraged as when they are angry AND ignored.
Now I would like to think the point was just evil action, but that might be the easy way out. The basic definition of terrorism is violence specifically to invoke political change through fear, but does appear effective in asymmetric warfare. But again, too simple.
Evil or not, getting the attention of a foe (or the public in general) psychologically legitimizes the attacker as a force that cannot be ignored. The metaphorical fly WANTS to be swatted, or they suffer a sort of non-existence, shouting into the wind.
For example, beheadings are great for effect because the news dwells on them. It’s no wonder Mexican Cartels beheaded women for YouTube after ISIS video went practically viral. The news coverage CAUSES it to be effective.
It’s the same for copycat killers and fake bomb threats. And in another tactic, the Unabomber’s Manifesto being published. And the murderers both toward their families and schoolmates nearly always have a hint of the killer experiencing alienation, dismissal.
But in most cases, if the media response wasn’t in turn amplifying and therefore glorifying, there’d be no payoff. Of course, 9-11 couldn’t be ignored or under-reported, of course, but before then, who could even find Afghanistan on a map? How many people even knew what the Taliban was? Talk about gaining street cred … geesh.