>> Maybe just a lot of nukes, carefully designed super-virus that wipes out our species, or sending a single self-replicating nanobot
The insane energy scales of relativistic travel (especially between .9 and .9999-repeating) combined with the ridiculously advanced technologies throws any speculation out of the window. Interstellar nukes surviving relativistic deep space travel while guiding themselves to a solar systems light years away (and with your ability to turn measure in tiny radians of the same)? Super-virus that operates across massive evolutionary scales while, again, surviving said travel (and for kicks only 240 of the known 500 amino acids are naturally occurring [1] and it depends entirely on random mutations and environment)? Self-replicating nanobots, one half of the holy grail (billions of years being the other) that would place in the hands of mortals that which allowed natural selection to give birth to the aforementioned aliens?
At the technological level of relativistic weapon systems + nanobots you're basically talking the ability to do whatever you wanted. If they can have near omnipotent self-replicating nanobots flying to destroy civilizations, why not to slightly shift their stellar orbit by consuming the rest of the solar system (relativistic guidance and navigation is an entirely different ordeal) to add some interstellar non-determenacy or send out nanobots to replicate their civilization on hospitable solar systems?
Sending out a bunch of relativistic nukes is probably going to be one hell of an expense (don't imagine you'd be able to do a gravitational sling shot without a black hole or perfect premonition). If the civilization was that scared of being wiped out, it'd probably be cheaper to build a single defense for their civilization rather than a bunch of attack platforms intended to wipe out other civilizations.
You are significantly underestimating what is possible. For one we are talking about a civilization millions of years older than us (very likely, the chance of intelligences evolving within a few thousand, let alone a few hundred, years of each other is negligible.) They will likely have hit physical limits of what is possible technologically long ago. Maxxed out tech trees, essentially.
It's possible they can have dyson spheres, and artificial brains the size of planets, running far faster and more efficient computers and algorithms. This is of course just wild speculation, I'm only trying to give a sense of how absurdly far ahead of us they could be.
Interstellar travel is difficult yes, but possible. We've had the basic technology requirements for interstellar spaceship since the 50's (project Orion.) If you accept the premise that it's possible to send a reasonable size package (possibly as small as a pin if nanotechnology is possible) then wiping out a primitive planet doesn't seem so difficult.
>If the civilization was that scared of being wiped out, it'd probably be cheaper to build a single defense for their civilization rather than a bunch of attack platforms intended to wipe out other civilizations.
Possibly. It depends entirely on the costs of defense against advance civilizations vs the cost of wiping out primitive planets before they become a threat. We don't know what futuristic interstellar warfare would be like, but my guess is crushing intelligences before they advance to far, is far easier.
Am I underestimating what is possible or are you under estimating what becomes possible when you spend those millions of years developing technology that could feasibly pull off any of those interstellar extermination methods?
The technology necessary to reach out and destroy another civilization light years away cannot be developed in a vacuum. No matter how you accelerate the nuke, navigate it or protect the payload from the harsh realities of relativistic flight, you will develop an entire canon of technologies and an understanding of the universe that before you would ever be capable of identifying, let alone destroying, another intelligent civilization.
The insane energy scales of relativistic travel (especially between .9 and .9999-repeating) combined with the ridiculously advanced technologies throws any speculation out of the window. Interstellar nukes surviving relativistic deep space travel while guiding themselves to a solar systems light years away (and with your ability to turn measure in tiny radians of the same)? Super-virus that operates across massive evolutionary scales while, again, surviving said travel (and for kicks only 240 of the known 500 amino acids are naturally occurring [1] and it depends entirely on random mutations and environment)? Self-replicating nanobots, one half of the holy grail (billions of years being the other) that would place in the hands of mortals that which allowed natural selection to give birth to the aforementioned aliens?
At the technological level of relativistic weapon systems + nanobots you're basically talking the ability to do whatever you wanted. If they can have near omnipotent self-replicating nanobots flying to destroy civilizations, why not to slightly shift their stellar orbit by consuming the rest of the solar system (relativistic guidance and navigation is an entirely different ordeal) to add some interstellar non-determenacy or send out nanobots to replicate their civilization on hospitable solar systems?
Sending out a bunch of relativistic nukes is probably going to be one hell of an expense (don't imagine you'd be able to do a gravitational sling shot without a black hole or perfect premonition). If the civilization was that scared of being wiped out, it'd probably be cheaper to build a single defense for their civilization rather than a bunch of attack platforms intended to wipe out other civilizations.