@edgarblythe,
The movie leaves a lot of questions if you think about it. If they were so anthropocentric and geocentric that they would unthinkingly destroy all life on other planets, then how did they survive themselves? Are they monolithic like ants?
Or maybe it's just us projecting an enemy as totally sociopathic as we usually do to an enemy that we are at war with.
As far as their inability to deal with Earth's microorganisms, I might say that we have the same problem ourselves. It seems like some microorganisms evolve so fast--especially viruses--that we have a hard time keeping up with them. .