Some of those civilisations may already be far more advanced, and have taken to the stars tens of thousands of years previously. No matter. The interstellar mass driver is a weapon against which no defense can be erected. No level of technology can track a 1-meter-wide iron sphere as it moves through interstellar space at near-lightspeed. If you're on the receiving end of a volley, the insanely blueshifted, tiny spec of light from the projectile won't arrive until shortly before the projectile itself. To an observer, it would appear as if all the planets in the system suddenly started exploding. Few systems would even be able to get a warning cry out, and those who did would be unable to provide actionable intelligence on where the attack came from. Even if they could provide actionable intelligence, their warning would spread out at light-speed. If there were listeners to receive it, then those listeners would also presumably have attracted the attention of the Fearers -- and a fusillade of projectiles would already be inbound towards them, almost immediately behind the warning itself.
Imagining such a scenario, the Fearers say to themselves: "If we don't do something like this, then surely it's only a matter of time before somebody else does. And we'll have no way to see it coming. So the first species to build interstellar mass drivers will, inevitably, be the last species left standing. Therefore, we have no choice but build and use them ourselves."
In other words, they do unto others not because they're bugnuts paranoid, but because they have a not-altogether-unrealistic apprehension that if they don't, others will do unto them.
This isn't Mutual Assured Destruction as we know it. What made MAD work was the fact that enemies could observe each other in realtime, and that retaliatory strikes would be possible. Neither is the case here. The Fearers are convinced that the only rational course of action is to assume others will attack someday -- if they haven't already -- and therefore their attack needs to be launched immediately.