A little research goes a long way when understanding terrorism:
All terrorist acts are motivated by two things:
Social and political injustice: People choose terrorism when they are trying to right what they perceive to be a social or political or historical wrong—when they have been stripped of their land or rights, or denied these.
The belief that violence or its threat will be effective, and usher in change. Another way of saying this is: the belief that violent means justify the ends. Many terrorists in history said sincerely that they chose violence after long deliberation, because they felt they had no choice.
Zionists who bombed British targets in 1930s mandate Palestine felt they must do so in order to create a Jewish state.
The IRA (Irish Republican Army) bombed English targets in the 1980s to make the point that they felt their land was colonized by British imperialists.
In the 1960s and 1970s, the Popular Front for the Liberation of Palestine felt that armed attacks in Israel were a justifiable response to the usurpation of their land.
Osama bin Laden's declaration of war on American interests in the 1990s stemmed from his belief that U.S. troops stationed in Saudi Arabia represented an abomination to the kind of Islamic state he believed should exist in the Arabian peninsula.
Uighur separatists in China today feel that Chinese religious repression (the Uighur Chinese are Muslims) justifies their terrorist tactics.
In some cases, people choose terrorist tactics based on a cause whose righteousness they believe in to the exclusion of nearly all else. Abortion clinic bombers in the 1990s and groups such as the Animal Liberation Front believe zealously in their causes.
Causes of Terrorism -- The Two Causes of Terrorism