"Green" aliens might object to the environmental damage humans have caused on Earth and wipe us out to save the planet.
This assertion relies on a number of truly staggering assumptions - that an race alien would care about environmental damage to someone else's
planet and that such a race would by virtue of existing be any better in that regard, for starters.
"There is something going on in time and space, and beyond time and space, which, whether we like it or not, spells duty." -Winston Churchill
"For of all sad words of tongue or pen, The saddest are these: 'It might have been!'" -John Whittier
"Apathy is death." -Kreia