The term 'terrorist' in the modern sense was used from the 70s and is related to warfare. A terrorist were either individuals affiliated with non-state organisations like al qaeda or state sponsored like Saudi Arabia & Iran. They are normally funded by private groups for non-state warfare purposes, deployed to cause economic turmoil. Recruited from destitute communities with the promise of paying their loved ones large sums of money after their demise. They are not mentally ill, they are soldiers doing a job. Think honour killing.