Most terrorism is considered an instrumental type aggression, as what majority of reading materials about "terrorism and aggression" suggest.
Instrumental aggression aims to hurt only as a means to attain some other end. Such aggression is carried out in order to achieve a specific goal.
It is made against another person in whom the aggression is used as a means of securing some reward or to achieve an external goal such as a victory. When one receives a reward because of deviant aggressive act, s/he will be conditioned towards committing that act again when motivated to obtain that previously possessed reward. - USLegal.com
Psychologists understand aggression to be behavior aimed at harming another member of the same species, and most psychologists distinguish between impulsive and instrumental aggression.
Impulsive aggression (also known as irritable, angry, or expressive aggression) is marked by strong emotion, especially anger, and is aimed at hurting another. Instrumental aggression is cooler and the hurt delivered to another is not an end in itself but only the means to some other end. See Some Things Psychologists Think They Know about Aggression and Violence to learn more.
For further reading, refer to SOCIAL PSYCHOLOGY By Arun Kumar Singh.