For anyone who has read the book, do they discuss the concepts of
dropping space rocks and aiming a relativistic vehicle at Earth? Both
scenarios would be relatively easy in comparison to, say, a full-scale
invasion of our planet by ships with lots of laser rifle-weilding aliens,
and we would have virtually no defense against such an attack.

Do I think an advanced ETI would actually do such a thing to a fellow
intelligent species? Only if they have some kind of version of Asimov's
psychohistory and predict that humanity may become a galactic threat
to them at some future date, or they picked up our broadcasts of Jerry
Springer and knew that something had to be done for the sanctity of
the Greater Universe.

We always assume that the Universe is full of conquesting aliens out
to take over and destroy Earth and humanity, but I often wonder if it
is we who will be the invading conquerers of the Milky Way one day?

If we do not evolve in any major capacity as we spread into space,
I can easily see our species "appropriating" other systems in the name
of manifest destiny. Even if the highest extraterrestrial life forms are
no "better" than the animals and plants on this planet, does that give
us the right to invade their domains in one form or another? What if
a colonizing ETI landed on Earth say 4 million years ago and decided
that those chattering primates in the trees of Africa did not qualify
our planet as being off limits to their galactic expansion? I can easily
see our ancestors (and we with an alien species) being disregarded
as not sufficiently advanced, especially if the ETI were not and never
were humanoid.

Or maybe that is the "destiny" of all life everywhere, to expand and
fill every available niche in the galaxy and beyond, and to push out
any competition, either on purpose or indirectly.