When terror is carefully modulated (an anonymous note; a small bomb that explodes just before a place of business opens; perhaps a tactful assassination) it often does bring an opponent regime to negotiations, or even collapse. In contrast, these horror-show attacks are bonding rituals for the perpetrators. They demonstrate to the perpetrators how ruthless they are and how implacable their dedication.
Terrorism is violent theatre, a bloody performance art depicting an interplay of public relations gimmicks and self-stroking pomposity subtitled with a moral superiority comment track. It signifies impotence and frustration at the gross unfairness of a world where love is rewarded with indifference, and indolent, egocentric brilliance earns only disdain and starvation.
Philosophy provides a corrective to the astonishment and tears at hearing of a violent terrorist action:
[W]e have already shown that every kind of power is included among the things which men desire, and that all objects of human desire are related to the good as the goal of their natures. But the ability to commit crime is not related to the good, and so it is not desirable. And, since every power should be desired, it follows that the power to do evil is not a power at all. From all this it is clear that good men have power, but evil men are weak. Likewise, the truth of Plato’s doctrine is evident; only the wise can do what they want to do; the wicked can follow their desires, but they cannot accomplish what they want. For they do what they feel like doing, and they suppose that they will find among their pleasures the good they are really looking for. But they are bound to fail, since shameful behavior does not bring happiness.
–Boethius, The Consolation of Philosophy, Book IV, Prose 2