something else that no body has considered...
Let's say you build the Terror Star, ok the galaxy learns of this via the GNN grapevine. your a bigger target now. Terror Star will be slow moving 1 - 2 spaces per turn. Terror Stars will have no defensive ability against a fleet of ships.

Now for the part that no one has thought about..

You reach your target planet to destroy. You initiate the attack and take it out. Now you have proven you will actually use this thing and your population will take a big dip in approval ratings like say 20 points. So if your hovering at 80% or less or like alot of folks 54% your now down to 34% or 60% etc.. and subject to the influence of the other races taking over more easily. The good alignment could get a +10 or +20 to influence against a race that has a Terror Star that has destroyed a planet. Nuetral Races will base their decision on whether the planet you destroy was good, neutral or evil and will side with the opposite. In the Case of Neutral Planet destroyed they would then just go to war with you right then and there they take it personally then..

I think terror stars are stupid myself.Any civilization capable of traveling from star to star in a couple of weeks would be able to destroy a planet alot cheaper than building a "terror star" simply by grabbing a good sized asteroid and propelling it up to a good speed towards the planet.Far cheaper and easier to do.

A more interesting thing would be the use of biological/chemical weapons to kill the pop without causing harm to the planet.

Care to research military grade RAID for use on the Torians?

In GC1 terror stars weren't the greatist thing...