Saint John Paul II explained the gravely dark and sinful nature of terrorism, and the Church’s proper response to it in his encyclical, Solicitudo Rei Socialis (#24.4):
“Nor may we close our eyes to another painful wound in today’s world: the phenomenon of terrorism, understood as the intention to kill people and destroy property indiscriminately and to create a climate of terror and insecurity, often including the taking of hostages. Even when some ideology or the desire to create a better society is adduced as the motivation for this inhuman behavior, acts of terrorism are never justifiable. Even less so when, as happens today, such decisions and such actions, which at times lead to real massacres and to the abduction of innocent people who have nothing to do with the conflicts, claim to have a propaganda purpose for furthering a cause. It is still worse when they are an end in themselves, so that murder is committed merely for the sake of killing. In the face of such horror and suffering, the words I spoke some years ago are still true, and I wish to repeat them again: ‘What Christianity forbids is to seek solutions to these situation by the way of hatred, by the murdering of defenseless people, by the methods of terrorism.’”