"Terrorism" in the broad sense of the act does not have a goal. Because it is a means to an end, not an end itself.
Terrorists do what they do for many reasons. Some are political, some are due to sectarianism. Sometimes its because they are crazy and have irrational motivations.
Rarely is it the goal of the terrorist to force a society into tyranny.
Timothy McVeigh was seeking revenge.
Bin Laden was trying to force the US to change its foreign policy.
No that's still an agenda. Nobody becomes a terrorist just because. There is always a motivation and goal. You say retaliation makes it an end to itself but I give as a counter Timothy McVeigh. He was seeking revenge and sending a message. Terrorism is a tactic used to achieve a specific goal.
No that's an oversimplification. I can ride a bike because I want to ride a bike. But terrorism by definition describes a type of tactic used to reach a stated goal. It is not simply killing people. That's called murder. And murder can be an end to itself. There have certainly been murderers who had a compulsion to just kill.
Terrorism is killing a lot of innocent civilians in order to incite terror in a population to bend to your demands. They have an agenda and feel they are justified.
Terrorists do what they do for many reasons. Some are political, some are due to sectarianism. Sometimes its because they are crazy and have irrational motivations.
Rarely is it the goal of the terrorist to force a society into tyranny.
Timothy McVeigh was seeking revenge.
Bin Laden was trying to force the US to change its foreign policy.
ISIS was trying to conquer the middle east.
Aum Shinrikyo were a doomsday cult.
So on and so on.