I don't think the Shivans blew up Capella just to keep us away from their space. That wouldn't really make any sense. All the Shivans had to do was send all those Sathanases into Terran and Vasudan space at the outset, and they could have finished us off easily. There was no need for them to go to such lenghts to deter us; they had already decimated half of the Terran and Vasudan fleets, so their victory would be guaranteed. No, I think Admiral Petrarch's theory from the final cutscene is more likely. The Shivans used Capella as a doorway to get somewhere far away, somewhere they couldn't access by normal means. Maybe, as the Shivan Manifesto hypothesizes, they were creatures of pure subspace and were trying to return to their realm. Or maybe they simply need to travel to another point in real space, perhaps even to a distant galaxy. Remember, the Sathanases were using some kind of strange subspace pulse on Capella. If they had simply wanted to blow it up, they probably could have simply used the equivalent of a really large bomb (think the Sun Crusher from the Star Wars universe). I believe that the subspace pulses allowed the Sathanases to use Capella as a massive subspace portal, much more powerful than the normal nodes. The Sathanases that jumped out were probably using this portal to reach their distant destination. Those left behind were sacrificed in order to sustain the energy necessary to keep the portal open. The Shivans wouldn't run for Gamma Draconis; they had no reason to, since the GTVA was all but beaten.