Sorry, but I'm still not getting the 'advantage'. This strikes me as one of those games where the only winning move is not to play.
Fortunately, the loss of its empire is a process that always seems to leave the nation in question better off, at least in modern times. As I see it the only possible justification for total galactic conquest is self-defense: do unto others before they can do unto you, more or less as you put it. Any advanced civilization that doesn't see that as a self-defeating justification is so different from my way of thinking that I couldn't possibly deal with them on any terms, offensive or otherwise.
Basically, what you're saying is that the only way to survive and prosper in the Universe is to be a murderous, psychopathic douchebag on a galactic scale. Not down with that, sorry.
Were these your words, or not?
You're projecting human flaws and attributes on an advanced race that would, by necessity, be almost nothing like humans. It simply does not follow that because the solar system has not (yet) been turned into grey goo, we must be alone in the galaxy.
Even if there were multiple civilizations in a galaxy, a single "defector" civilization would have vastly more matter under its control than civilizations that did not use Von Neumann probes to expand. With such a huge advantage, they would quickly destroy all competitors.