A couple of the worst deeds done by aliens to humans in film
Ruined dinner
No one who saw the creature emerge from John Hurt's stomach in Alien (1979) had much of an appetite for at least a week afterward, although Spaceballs did succeed in making light of the scene eight years later by having the baby alien do an impression of Michigan J. Frog.
Screwed with our heads, literally
Tim Burton's mischievous but brainy aliens in Mars Attacks! cut off Professor Kessler's (Pierce Brosnan) head, but manage to keep him alive. Later, in Burton's twisted idea of a romantic turn, Kessler falls in love with Sarah Jessica Parker's character, whose head gets swapped with a Chihuahua.
And a couple good things they've done
Made Reese's pieces huge
E.T.'s appetite for this candy in the 1982 movie arguably led to the eventual peanut-butterization of most candy bars, which, if you ask me, is a very good thing. Of course, it also proved the enormous effectiveness of product placement, which is good if you're getting a cut from the deal, bad if you have any artistic integrity. Blew up the White House
The squid-like aliens in Independence Day, who have mean flying-saucer skills, manage to enter White House airspace and blow the president's mansion to smithereens. Just kidding, that's not a good thing at all, is it.