What is more important in determining terrorists’ motives: the psychology of the individual or the sociological impact of the group?

I feel it is more important to look at the psychology of an individual when determining terrorists’ motives. If you can figure out the why they are doing these things then you can determine what type of sociological impact they are aiming for. Thus determining their motives for their actions.
In chapter 4 of Terrorism in the Twenty-First Century, Cindy Combs talks about why people become a terrorist. She talks about how the individuals identify with the leaders, members of groups, and also with those that have suffered. These individuals then feel the need for revenge and avengement of the wrongs done to them. On page 67 of the above mentioned text in a statement from Osama Bin Laden on Al Jazeera he explained how thankful he was that America was now feeling the same pain that the Islamic people had been feeling for 80 years. This shows that revenge was a major factor in his actions. Combs also talks about (pp73-74) how the individuals view the world. That terrorist are now coming from low income areas because it is their only way to lash out and respond to the government/ world around them.
In The New Global Terrorism (p99) Martha Crenshaw talks about individual motivation and participation. She explains how the individual’s views can justify terrorism. Such as it they see the government corrupt and unjust then they can justify their acts by saying it was an only option type scenario. She also explains how the risk for personal danger is a shared trait among terrorists. The followers also share a psychic gratification from committing the acts. The leaders, more like those who plan the attacks, are aware of the risks as well. They both feel that their commitment to the cause is ranked higher than their personal risks. On page 103