Dr. Frederick J. Hacker once characterized terrorists into three groups: Crusaders, Criminals, and Crazies. Crusaders fight (and kill innocent people) because they believe that they are doing it for a cause - freedom, liberation, acknowledgement, etc. Some might even call Al Qaeda a group of Crusaders. Criminals do it to make money - drug dealers, mafia members, that kind of thing. Crazies do it for a simple reason - they're mentally deranged people.

There are many types of terrorists, many different people. Their personalities obviously differ, and there could be many kind terrorists - they would almost all go under the 'Crusader' category. However, they still use violence to solve their problems and to cause havoc.