MEGATRON
The origin of Megatron is pretty well non existent, the only thing that even resembles an explanation is revealed to Rodimus Prime when he short circuits himself and goes on a bit of an Out of Body Experience inside the Matrix of Leadership in the episode “Five Faces of Darkness pt 4 “ to try and get an answer as to who the Quintessons were.
Whilst inside the Matrix, Rodimus is shown scenes from Cybertron’s past from the memories of the ancient Autobot leaders held within, they explain the creation of Cybertron by the Quintessons as a factory to manufacture two product lines of robots, one for consumer goods and one as military hardware.
These two product lines were to become the Autobots and the Decepticons respectively and how after much abuse by their masters, evolved into feeling beings and overthrew them sending them fleeing off world.
After a period of peace war broke out amongst the two factions which lasted many centuries, the Decepticons out gunned the Autobots with their greater firepower so the Autobots turned to stealth and developed the “art of transformation“, this brought the about the end of the war and the rule of the Autobots began.
Howoever many centuries later the Decepticons also developed the ability to transform and “thus one fateful night, far below the city Megatron was born “
Megatron went on to kill the last of the primes and then years later shot and fatally wounded dock worker Orion Pax during an energy raid at the warehouse he worked unwittingly creating his own nemesis, Optimus Prime ( whoops ) and starting the 3rd Cybertronian war.
In the G1 cartoon, Megatron is given his lovely singing voice by Frank Welker, Frank also voiced other well known characters such as Soundwave and Skywarp, as well as various aliens and creatures.
He also voiced Megatron in Transformers : The Movie ( 1986 animated ), Devastation : The Game, Revenge of the Fallen : The Game, The Ride 3D, LAst Knight, Transformers Prime and Transformers Prime : The Game.