Honestly, there's no real reason to assume that because a species may have achieved interstellar travel that they must be more enlightened, or moral, or whatever way you choose to describe it, than we are. I mean you could argue some relative stability on a planet would be required to get to that level of technology the way we did, but even if they were a race at peace with themselves, there's always rampant xenophobia for them to fall back on. Who's to say that they would even recognize a species like ours as life at all (or at least a comparable form of life), let alone as intelligent, sentient beings? They may believe their species is the only life with value and just start a genocide from one end of the cosmos to the other.