I'm sure this has been discussed before, but I don't visit the boards too much, so sorry if I'm beating a dead horse. But in the movie universe, it seems like the Decepticons goal is to carry on their race. They are motivated by the natural instinct of survival of their kind. I mean, yea, the Fallen just seems to enjoy being a dick, but besides him they seem to have the Cybertronian lineage in mind to explain their intentions. Or maybe they are just all dicks.