Why do we always assume that extraterrestrial visitors come from a wiser and more benevolent planet than our own.
They could come from an evil and malevolent planet, planning to end our lives and reduce it to minimal level so that they can extract anything in our planet worth having - minerals, plants, animals and then send whats left heading into the sun and total annihilation.
Or they could be refugees from a another planet so incompetent that they have destroyed it and having reviewed all other planets decided that we have done better than any of the others when it comes to dealing with the problems, that we, like they, have created.