I can think of a few simple (maybe simplistic) reasons for the dichotomy: * We imagine theoretical "people"/beings to be good, but are confronted with the comp

I can think of a few simple (maybe simplistic) reasons for the dichotomy: * We imagine theoretical "people"/beings to be good, but are confronted with the complicated reality of our neighbors. Non-human entities remain strictly theoretical, untainted by the same reality. * We see spacefaring beings as advanced beyond ourselves, and aspirationally believe that goodness accrues with advancement. Or at the very least, that our trivial little human problems are beneath the concerns of sufficiently-advanced societies. * We believe that much human conflict is driven by resource scarcity, and that a spacefaring culture surely does not lack for resources. The opposite conclusion can be drawn from one of the other major drivers of human conflict: religious/etc ideology. We humans have no (widespread) ideology that is so nihilistic as to advocate for the elimination of all humans, but we are pretty good about constructing ideologies that would eliminate large percentages of other humans. If we made contact with another civilization (or were made contact with), I would be shocked if some major religion didn't declare the aliens to be apostate/devilspawn/etc -- because how else could a non-Earth-based civilization fit into the mythology?