I would define terrorism as forced coercion of an outside group (or group perceived to be "outside") through violent actions. Controlling through fear. So yes, any assassination, and especially that one, would be terrorism. However, terrorism committed by independent actors makes them terrorists, but terrorism committed by the state is more appropriately classified as military aggression, or acts of war.

The whole point of the military doing anything short of all out war is to scare an "enemy" into compliance through threat of violence. So it's not classified as terrorism for the same reason as lethal action by police is not murder. The blessing of the state changes its definition.