Unless something about life is fundamentally adversarial I wouldn't expect civilizations to be hostile to one another by default. You could make the argument t

Unless something about life is fundamentally adversarial I wouldn't expect civilizations to be hostile to one another by default. You could make the argument that it does appear that life is fundamentally adversarial, seeing as in our one example all life feeds on other life, and to sustain life a creature must kill. I believe that's simple a circumstance of limited biomass in a closed system and so not fundamental, but you could argue that all civilizations probably get bootstrapped in this way and so it doesn't need to be fundamentally adversarial, but that all civilizations are accustomed to it so the distinction "fundamental" does not really matter. I'd disagree with that also, because a change in conditions to post scarcity changes conditioning and therefore behavior. You might also come up with a prisoners dilemma type scenario where a civilization doesn't want to be adversarial, but it doesn't know the intentions of others and so to protect itself, as a rule, destroys any that it finds. This is somewhat plausible but I don't really think likely because civilizations that evolved from abiogenetic origins separate from one another would likely have very very different behaviors that might not even overlap in many cases, so even if they were aware of each other they may not even be capable of interacting. It just does not make sense for a space faring civilization to be hostile. A requirement for space faring and a result of it is post scarcity; resources abound in the vast swathes of dead universe. There is no reason to even think competitively, barring some as yet undiscovered unobtanium so rare and so powerful that civilizations fight to extinction over it, but I doubt it. If anything, I'd think that a space faring civilization benefits the more it understands the universe, and learning about biospheres and civilizations with completely different evolutionary paths from abiogenesis would be more valuable than destruction of them.