Terrorism was originally theorized in the context of insurgency and guerrilla warfare, a form of organized political violence by a non-state army or group. Individuals, abortion clinic bombers, or groups, like the Vietcong in the 1960s, can be understood as choosing terrorism because they don't like the current organization of society and they want to change it.
Saying that a group has a strategic cause for using terrorism is another way of saying that terrorism isn't a random or crazy choice, but is chosen as a tactic in service of a larger goal. Hamas, for example, uses terrorist tactics, but not out of a random desire to fire rockets at Israeli Jewish civilians. Instead, they seek to leverage violence (and cease fires) in order to gain specific concessions related to their goals vis-a-vis Israel and Fatah. Terrorism is typically described as a strategy of the weak seeking to gain advantge against stronger armies or political powers.
Research into the psychological causes that take the individual as their focus began in the 1970s. It had its roots in the 19th century, when criminologists began to look for the psychological causes of criminals. Although this area of inquiry is couched in academically neutral terms, it can disguise the pre-existing view that terrorists are "deviants." There is a substantial body of theory that now concludes that individual terrorists are no more or less likely to have abnormal pathology.
Group Psychology / Sociological
Sociological and social psychology views of terrorism make the case that groups, not individuals, are the best way to explain social phenomena such as terrorism. These ideas, which are still gaining traction, are congruent with the late-20th century trend toward seeing society and organizations in terms of networks of individuals. This view also shares common ground with studies of authoritarianism and cult behavior that examine how individuals come to identify so strongly with a group that they lose individual agency.
Socio-economic explanations of terrorism suggest that various forms of deprivation drive people to terrorism, or that they are more susceptible to recruitment by organizations using terrorist tactics. Poverty, lack of education or lack of political freedom are a few examples. There is suggestive evidence on both sides of the argument. Comparisons of different conclusions are often very confusing because they don't distinguish between individuals and societies, and they pay little attention to the nuances of how people perceive injustice or deprivation, regardless of their material circumstances.
Career terrorism experts began to argue in the 1990s that a new form of terrorism fueled by religious fervor was on the rise. They pointed to organizations such as Al Qaeda, Aum Shinrikyo (a Japanese cult) and Christian identity groups. Religious ideas, such as martyrdom, and Armageddon, were seen as particularly dangerous. However, as thoughtful studies and commentators have repeatedly pointed out, such groups use selectively interpret and exploit religious concepts and texts to support terrorism. Religions themselves do not "cause" terrorism.