I think killing-oppressors, is an overly simplistic view of aliens.
and it's actually only ONE of the narrow possibilities where the Alien's space faring technologies are achieved radically before they evolve to grasp at desires beyond merely conversion of mass into one's species.
Let's say they achieve space faring technologies early, IF they're still capitalist , aka , self hating juvenile youth, Then that technology is far more dangerous to THEMSELVES than anyone else.