Here are some crucial signs that we might have totally misunderstood what an alien has in mind.
The alien might just be here to warn us.
In The Day The Earth Stood Still, Klaatu comes in peace — but we still shoot him and generally treat him like crud. Turns out he’s here to warn us against taking our aggressive, warlike tendencies into space — the same tendencies we just demonstrated.
It might be protecting its young
The Horta in Star Trek is just minding its rocky business, until the miners start digging their tunnels and hurting its rock babies — and then the Horta has to defend its offspring.
They might not be attacking after all
Babylon 5’s Minbari are sort of the poster aliens for “appearing to attack but not actually attacking” — that’s how the big war got started, and it’s also how they try to provoke Commander Sheridan into a fight on his first day.
They might be innocent
In Joe Haldeman’s novel The Forever War, the Taurans are a clone species who can’t communicate with the non-clone humans — and they get falsely blamed for destroying some colonial vessels, touching off a war that lasts a thousand years.
They might not have thought we were intelligent
The Formics don’t believe that humans are an intelligent species — until we start wiping them out, in Ender’s Game. And by then, it’s too late to talk it out.
We made a deal with their enemy
Good old Captain Janeway — she’s always happy to make some crazy deals to help the Voyager survive. Sometimes she makes deals with aliens, just for the rush of day-trading deuterium. Except when she makes an alliance with the Borg, it winds up getting her on the wrong side of their enemies, Species 8472.
They might just want to meet us.
In Contact, the aliens send us the plans to a travel device, so we can come say hi. But many humans fear that the device could just as easily be some kind of Trojan Horse. We build it and out pours the entire Vegan army.” (And yes, Vegans are scary.)
They might be just passing through
That’s the case in E.T.: the Extraterrestrial. He’s just lost and wants to go home. We also get to see that experience from the human POV in the Farscape episode “I, E.T.”
They just want some of our natural resources
In The Man Who Fell to Earth, David Bowie just wants to ship some water back to his dying home planet — but the evil earthlings lock him up and do tests on him, plying him with booze until it’s too late.
It could be hunting another alien who’s the real problem
There are lots of examples of this happening in science fiction, but one example that comes to mind is the 1950s pastiche movie Alien Trespass, where an alien cop’s prisoner, the evil Ghota, escapes. The cop has to hunt down the Ghota, but he’s hampered by the cops, who don’t believe his intentions are benign.
They could just be trying to be inconspicuous
In It Came From Outer Space, some aliens have crashed on Earth and they take control of a few humans to help them get around, since they can’t move among us in their true form without causing a panic. The aliens just want to repair their ship and leave, but the humans notice that people are disappearing and assume the aliens are kidnapping them for evil reasons.
( via io9.com )