They would be cutting their own throats by destroying the resources they rely upon. The greatest danger arises from political groups with a single focus or cause. Extremist religious groups also pose a serious threat. It takes a very strong belief system to become involved in causing serious bodily harm and property damage. Those open to other ideas and willing to consider other perspectives don't kill on an offensive basis.
Absolute intolerance produces the heinous crimes of mass destruction.
The dilettante and the emotionally soft will not embrace terrorism. So being able to assess threats as to an adversary's capabilities and intentions becomes critical to survival. The intelligence arm of computer security must understand psychological profiling and possess a firm knowledge of the assets available to various extremist organizations. Understanding the enemy's capabilities allows for instituting countermeasures when appropriate and exercising other options to minimize exposure. The first step in the preparation of actionable intelligence requires a realistic appraisal of what damage cyber-terrorism will cause.