Why do organizations engage in terrorism?
Why do individuals join terrorist organizations?
Why terrorism instead of politics, art, religion as a tool of transforming society?
Here's a definition of "terrorism": extra-legal violence directed at targets that not primarily military, military-industrial or political with the purpose of the violence being to create fear and otherwise disrupt society. The violence has a political agenda and is planned and executed by an organization. Violence by the powerful is categorized differently by conventions of common use.