There are many lofty, high minded goals one could consider; exploration, discovery, species/ecosphere survival in the face of an imminent global threat. Finding, enslaving, or eliminating other sentient life forms? I can't see how this could be the goal of any civilization. Maybe the same impulse that could lead to organizing such an incredible, global effort to build the interstellar infrastructure in the first place could also dictate this less savory motives, ie religious zealotry ie Church of Galactic Subjugation.
The above is speaking to a possible interstellar civilization and not much more than our tech level.
Some points I like: - A very advanced lifeform may view us the way that we view plants, so basic that wiping them out for convenience sake is not even something that enters the realm of our ethics - we just do it. I imagine aliens coming to Earth and seeing us humans on it eliciting a response like one of us opening a container in the fridge and going "Eww mould" (ewww Primates) and then washing it all clear with hot water.
- One would think that extra-species aggression would be a trait favorably selected by Darwinian evolution, so super-hostile aliens would survive and propagate. Hopefully that's negated by a post-scarcity advanced alien economy?
- In the universe sentience is relatively rare, so finding another race is something that should be treasured, cherished and nurtured.
- Maybe they'll see us as pets, and shower us with gifts
What other scenarios can you think of? I especially like thinking about what happens when there's large intellectual differentials ...
... unless you consider that all life on this planet began with the same chemicals, and that many different organisms are codependent, the most obvious being plants and animals.
Truly, interstellar existence, once confirmed that we're not alone, may also require teamwork. Perhaps ET is just waiting for us to demonstrate that we don't intend to destroy our planet for the benefit of a few selfish short-lived creatures in Earth's long history.
I have heard this line of reasoning often, but it's not clear to me that the logic is correct. I don't think an entity's treatment of itself necessarily implies much about the way it will treat others.