At the end of the movie, when Sam is running towards the rundown building to get the cube out of the city, he bumps into Cadillac Escalade and drops the cube onto the ground.
The xbox 360, the mountain dew vending machine, and the Cadillac received the spark from the cube and therefore became robots.
Now, did anybody noticed that the robots, as soon as they were given life, they started to destroy humans? I think this kind of gives a hint that suggests why Megatron wanted the cube.
Also, this is just my opinion, but I think that when Cybertron was created, all the characters from autobots were not necessary evil, but violent and destructive.