Are you asking about the 3 body problem version of this? Spoiler alert: The folks doing the eradicating aren't spending much time/energy/anything on eradicating. It's one large missile through space.

I think the gist is: sure, we humans can't conceive of getting to anyone else in the universe in any timescale, but if we can keep ourselves from destroying ourselves, we'll eventually figure it out. And we'll spread. And we'll kill everything that isn't us in the process as we've done as explorers on this planet.

So really in 3BP: it's inexpensive to eradicate. But insanely expensive to possibly get the intention wrong of any other civilization you encounter. They might kill you.

(again, this is just my interpretation of what 3BP said)

I don't think it's correct that we destroyed everything that isn't us. If we take all living beings, we have destroyed only a small percentage.