The Decepticons are one of the primary factions in the Transformers mythos. They are typically concerned with such things as conquering Cybertron, defeating the Autobots, amassing large quantities of energon, developing powerful weaponry, and beating people up. Not necessarily in that order. Unlike the Autobots, whose leader is a Prime bearing a Matrix, the Decepticons are led by the most powerful of their ranks. This tends to cause some conflict, given how generally every Decepticon thinks that they're the most powerful.

Also, the Decepticons are not exactly the most compassionate beings in the universe, but not all fight for greed. More than a few have a sense of honor, while others believe that Cybertron would be better protected by aggressive expansion. Ultimately, the Decepticons desire to protect their homeworld, even if it is at the expense of others. Usually.

Megatron

Megatron is the founder of the Decepticon uprising, and their most well-known and feared leader. As a young, charismatic leader forged in battle and the heritage of war, he began to believe in a grand purpose for his race—it is Megatron's glorious destiny to rule an empire which will span the universe. The opposite of his mortal enemy Optimus Prime, he feels great contempt for other Transformers who, he feels, betray their proud heritage by demanding peace and cooperation with weaker life forms. It is the destiny of the Decepticons to bring order to the universe through conquest, though in the millions of years since coining this purpose it remains to be seen how much of his mission statement is altruistic... and how much of it is mere words built to fuel warriors to further his desire for personal power.

Megatron will attempt almost anything to achieve his goals, but his schemes are rarely in any way subtle. This is perhaps due to his arrogance, but his pride is not so strong as to dissuade him from abandoning a battle he is losing. He often shows dramatic examples of cowardice, concealed in rage.

Although he isn't as physically strong as Optimus, his ability to analyse any situation and utilize it as a weapon makes the Decepticons deadly, so they can find energy sources almost immediately. Some would question his sanity, though these few are mostly now dead by his hands... or his fusion cannon, depending on his mood. Sometimes he uses his energon mace to strike them down. It does not matter how they die. Death by the hands of Megatron is an honour.

"You destroy everything you touch, Megatron!" -Optimus Prime to Megatron

"That's because everything I touch is food for my hunger. My hunger for power!" -Megatron to Optimus Prime