Brilliant movie idea...
Starts on a peaceful, Medieval-esque planet filled with aliens who believe in hope, love, and collective teamwork. Suddenly, one day, their telescopes see a group of objects hurtling through starry space. Eagerly they await the coming of these crafts. Long have they wondered if they were truly alone in the universe. Now they know. An advanced race, a thousand years ahead of them, is coming to grant them wisdom and knowledge beyond their years.
As the first ship floats down into the planet's capital city, swarms of happy civilians watch it land. As it touches down, the Grand Leader appears, ready to welcome the visitors. As he approaches the strange object, he is blinded by a great light as the vessel opens up. Out of the corner of his eye he sees the being in the doorframe, a muscular individual with two arms and two legs, just like them. He seems to be wearing some kind of armour, but that is probably for his own protection.
The Grand Leader walks closer, outstrenching both arms in greeting. Before he touches the being, however, he is kicked down the stairs. He looks up, and sees the visitor is holding some kind of cylinder pointed at his face. There's a flash...and then nothing. The visitor continues, firing some kind of unspeakable magic at the crowd, destroying the creatures left and right...it's like Mars Attacks, but we're the bad guys, awesome!