If aliens wanted to rid our world of life to get our resources, what would be the most efficient way to go about it?

I'm guessing they'd just pump Hydrogen Cyanide into the atmosphere from low earth orbit...

Or would that contaminate our resources so they couldn't take them?

A systematic bombardment of all the cities, ala independence day, just seems grossly inefficient... that movie would of been alot different if they just started pumping cynaide into the atmosphere while we didn't know about them... don't you think?