The decepticon movement was clearly around before Megatron was created.I have always wondered who was Decepticon leader just prior to Megatron.Did he kill be previouse leader and then asumed the role of leader.
shareAs far as this cartoon is concerned, Megs was specifically created to be their leader after this guy got killed by Sentinel Prime:
http://tfwiki.net/wiki/Early_Decepticon_leader
I was always under the impression that Megatron was the founder of the Decepticons and thus became their leader since he started the movement in the first place. He rose to power kinda of like Napoleon Bonaparte and Alexander the Great did on his own, and formed the the Decepticons, kinda like how Genghis Khan did with the Mongols, or Hitler with the Nazis and the Third Reich.
But then again there may have been Decepticon leaders before Megatron, since at Starscream's coronation there was a Pantheon of statues surrounding the ceremony hall that might've represented past Decepticon leaders. http://tfwiki.net/mediawiki/images2/b/b1/Hall_of_heroes.jpg
In fact there were also Decepticon leaders after Megatron/Galvatron. These include Lord Zarak with Scorponok, Overlord, and Deathsaurus.