The way I see it, it's all about conflict and evolution through conflict. First we have the Ancients, a master race in the galaxy, top dogs of them all. They go about exterminating anyone they can find. Superior technology gives them the edge they need to prevail continuously. Then, one day, they meet a species that gives them a bloody nose. Insulted, surprised and angered, they come back with even greater a force. Eventually, this attack is rebunked as well. The Ancients are in a situation they have never encountered before, someone is actually putting up a fight.
This new race, the Shivans, eventually grow stronger than the Ancients. The Ancients are driven back, defeated for the first time in their history. We do not know if any communication took place between the Ancients and the Shivans. Anyway, no peace treaty was ever signed. Given the history between the species, the Shivans see it best to exterminate the Ancients, once and for all. They are too great a risk to all the life in the galaxy.
Strengthened by the conflict, Shivans are now number one race in the galaxy. They explore, they expand, they exploit. During their excursions, they find new species, species inferior to them. We don't know the reason why the Shivans decide to exterminate these species. Perhaps they wanted to make sure that there would never be another "Ancients". In any case, the Shivans begin a policy of genocide. With their technology and combat experience, they are invincible.
Until one of their fleets discover two civilizations at war with each other. The Shivans roll in, expect another walk-in-the-park military campaign. They are, however, in for a nasty surprise. These Terrans and Vasudans somehow manage to destroy the invincible Lucifer. At last, the Shivans have found a worthy opponent. The gloves are now down, and the Shivans begin to assemble a new and stronger attack fleet. No species have, as of yet, managed to turn back a Shivan invasion.
32 years pass without encounter. Then the Shivans meet their enemies once again. They have grown stronger, a lot stronger. The Shivans can draw comparisons between this conflict and their conflict with the Ancients. They decide that this threat is to be eliminated now, lest it be too late. They send in the Sathanas, indestructible by any known GTVA weapons.
To horror of the Shivans, even the Sathanas is not enough. The Shivans now realize the seriousness of the situation. Should this GTVA ever reach their home systems, there would be no mercy given, no Shivan colony left standing. They decide to send in a fleet the galaxy has never seen before. The GTVA has grown to match the Shivans. No punches will be pulled anymore. Only one can win this war, and the victor is the one who survives.
True enough, the Shivan assault drives the GTVA back from the nebula, then from Gamma Draconis, all the way to Capella. The Shivans have now the upper hand, the GTVA is beaten, the Colossus is nothing more than dust. Shivans see the desperation in the eyes of their enemies. As a last resort, the GTVA starts collapsing the subspace nodes leading to its space.
However, something else has caught the Shivans' attention. One of these GTVA ships is acting strangely. They realize it is trying to communicate with them! Initially, the Shivans are very wary of this communication attempt. They fear this GTVA, more than anything in the galaxy. Eventually, they decide to meet this envoy. We can only guess what discussions took place during that meeting. Perhaps Bosch went willingly with the Shivans. But in order not to raise suspicions, they made it look like as if he was captured. Possibly the GTVI spied this conversation, we don't know what it was that Snipes was rescued for.
We can assume that the destruction of Capella was a direct consequence of these talks. There is no other rational explanation for that kind of mass suicide, given that we know nothing of the Shivan society, their motives or their beliefs.