One thing which all such scifi flicks have wrong... The cost of interstellar travel would dwarf the value of anything you could hope to take or steal from another star system. The ONLY rational motives there could be for interstellar travel would be knowledge, or escape.
Food or living space come to mind.
Or they need a new home. In which case, we'd be in their way and would be quickly eradicated.
Stephen Hawking has made a pretty compelling argument that maybe we shouldn't try and contact alien civilizations. It wouldn't be anything like Columbus and native Americans. It would be closer to humans vs. ants. When was the last time you tried to engage in diplomatic negotiations with that fire ant hill in your back yard? You didn't. You eradicated it swiftly without any troubling thought.
This is what aliens would do to us.
“TO SERVE MAN”
It’s a COOKBOOK.