Even at our worst, humanity never tried to kill all of a new group that they encountered. Trade and diplomacy (admittedly with a wide range if violence underpinning it) has always been the human M.O. with inly a handful of very rare exceptions.

Even European genocide of Native Americans started off with intentions of trade. In fact, it was really only British colonization, carried on by their American and Canadian descendents, that opnelt tried to exterminate the natives. French colonization (outside of Caribbean slave plantations) was mainly focused in trade networks centered around frontier outposts. Spain came in a toppled the existing power structures in Meso and Southern America and ruled over the deposed empires.

Encounters with advanced alien races wouldn’t result in the intentional destruction of humanity, but instead would likely end up with total upheaval of our society, and the powers that be are terrified of that.