As it comes around things in the galaxy aren't always as one would think. When one worlds problems crash down on another things change and people as well as those others change as well. When an Earthquake rocked the coast of Oregon and awoke a sleeping Volcano for a fleeting moment the small eruption disturbed the occupants of these that had been resting there after four million years. Those that proceeded to emerge came to a planet thriving with a species called humans. The Transformers as they called themselves broke into two warring factions, though to the human race most did not comprehend the differences. The first ones that made themselves known were the Decepticons; choosing to build a ship, and return to the planet which they came. The second one, the Autobots, took it upon themselves to see that the more violent Decepticons did not hurt the occupants of the planet called Earth.
Several years passed as well as several campaigns to 'handle' the Decepticon threat which was always being deliberated on came and passed. World leaders unable to reach a real agreement considered handing the Decepticon Leader Megatron the Energy he craved to persuade him to leave Earth while Optimus Prime Leader of the Autobots tried to evade that action by explaining that it would never be enough.
Autobots would battle their foes the Decepticons while humans who were not always innocent died in the crossfire. Decepticons not caring for humans or the meager facilities they created took what they desired from human and Autobot alike.
One might be lead to believe that the Decepticons were the ones that were on the side of Evil. Sometimes that is not always the case. One will steal a loaf of bread to feed their starving family if one has to, doesn't make them evil, just broke. Not that a Decepticon would take the time to inform you of such things, but in the grand scheme of it all it was very much this concept on Cybertron. Once the Decepticons on Earth reestablished contact with their home planet the guardian that over saw the Decepticon ruled areas of Cybertron informed Megatron of its status and lack of energy. Megatron already know when he had left that the planet itself was in need. The wars that had taken place over so many millennia and more had taken its told. His officers informed him that the Autobots had built a ship to explore and find energy, and in the Leader's optics, they had done just that. The Earth is rich in various resources, even the rays of our yellow sun can be used to create power. Decepticons if they are anything, are not stupid. Megatron took advantage of what was at hand and when the Spacebridge was established began to ship loads of Energon back to Cybertron.
Realistically the Decepticons were not stranded on Earth, Megatron choose to stay and send the energy harvested from this planet back to Cybertron. The question really is why? Some will say greed. Others will say selfishness, while yet others may even go as far as to say replenish Cybertron. Most of those that have come into contact or have lost a loved one to the many battles, don't care; they just want them to go back to where ever it is they came from.
How would you feel? Kill a few ants to save your picnic? Steal a loaf of bread to feed your child? Save those that cannot save themselves? Perhaps it is your turn to make the choice and find out where you stand? In the process perhaps the question of why may be answered, or even a darker side of yourself reviled.