Hmm.
Going back to the idea that Shivans are a mechanized race:
It makes a lot of sense if their creating race needed specific conditions to thrive, such as a lack of certain wavelengths of EM radiation (they might sunburn easily/lethally, who knows). Now given that only species that are selfish are viable from an evolutionary standpoint, it makes sense that as soon as they have achieved sufficient technology to leave their home planet (which is probably shrouded in an environment that blocks a lot of light from killing them), that they would want to. Except, they realized that leaving their home killed them. Yay.
So, they still want to leave...overpopulation and global warming are screwing up their home. So, what do they do? They build a fleet of genetically engineered cyborg creatures called Shivans that serve one purpose: Find or create environments for their masters.
Because most of a planet would have to be mined empty to create these fleets, the master race realized that they probably didn't want some other life form coming across them and converting their centuries of work into pretty clouds of debris. So, they arm them. They also didn't really care about negotiations or any of that junk, because they're concerned about self-preservation, not harmonious existence, therefore, they genetically program the Shivans to blow anything up that doesn't carry master race identifiers.
So, the Shivans are produced from the master race shipyards, and are warped out to random spots in space to either find or create nebula environments that would block out the appropriate wavelengths of light that are lethal to the master race, enabling them to spread out their territory.
Then comes Bosch, who uses ETAK to communicate with Shivans. The Shivans must also have used quantum pulses to talk with the master race (since that's all they can use), then mistake Bosch for either a master race individual, or someone who might be friendly with the master race, so they take him in but kill all the rest of the stupid humans that can't possibly be buddies with the masters.
Therefore, another possibility is that the Shivans just roam the universe blowing stuff up and making nebulae from suitable stars whenever they come across one. In a way, you could argue that they're even more hegemonic than the Ancients.
Or, we go with the Xel Naga parallel, which is that the Shivans completely owned the master race after developing sentience accidentally or something, and now just roam around with broken genetic programming that just destroys stuff that would have benefited the now extinct master race.