The terrorist either obtains control of the system or alters the system in such a fashion that airplanes are flown into each other, resulting in mass death. The computers used in the air traffic control system do not control anything. They merely provide an aid to the human controller. A basic tenant in pilot training is situational awareness . From the first day of training, pilots are taught to be aware of not only their location, direction and altitude, but those of all other aircraft.
Pilots routinely catch errors committed by air traffic controllers.
It is the spectacular human failures that result in aircraft collisions. Further, the rules of the road for aircraft operations anticipate the complete failure of the air traffic control system. A similar scenario is proposed concerning the operation of a subway or train system. Brief reflection will show that failures of the electronic and mechanical controls are anticipated. That is why few of these systems are not manned .