If humankind can get itself off the planet while still being assholes, so can aliens. Long and tedious alien bureaucracy, death ray travel times or pure sadism in watching humanity suffer might be the only reason we still exist. So the moment people get a solid foot in space without losing the idea of mitigating potential future threats, people become dead/enslaved. If humans get to space being hippies or some shit, there's a tiniest chance everybody else did the same, and then there's galactic raves, bad music, drugs, peace and sex for everybody and their pet robots. On the other hand, that tiniest chance will still exist if we stay warmongers, and then we get a chance to raid, rape and demolish a whole fucking galaxy of defenseless hippies. And genocide is certainly one of our favorite hobbies.