If you were to ask pretty much anyone who the leader of the "bad guy" Transformers is, they'd probably be able to tell you: Megatron. However, if they're not able to tell you very much else about the Decepticon leader, they can probably be forgiven. It can be hard for those of us who are known for being Transformers fans, too!
When the character was first introduced 30 years ago, there were (broadly speaking) two ways you might learn about Megatron: the Marvel comics and the cartoon. The Marvel comics were of limited help, at best. Megatron only held the position of Decepticon leader for roughly 10 issues of the entire 80-issue run (he did have a few more appearances than that, but it should be noted that he didn't always appear in issues during his run as "leader," either)! While he appeared in far more episodes of the cartoon (during the first two seasons, in any event), I'm not sure one could say more about his personality than "stereotypical evil megalomaniac." Almost every episode, he'd come up with another scheme to take over the world, but one didn't have to be the Brain to recognize the vast hole in nearly every plot.
And this guy is said to be the fearsome Megatron, whose very name is supposed to cause all other Transformers to tremble?
While some early comics (especially in the UK) tried to flesh out Megatron's history by making him a formidable gladiator who rose to power to lead an army, it really was left to more modern comic writers to give Megatron real motives for his tyrannical ways. Megatron, we eventually learn, was a miner during a period in Transformers history when corrupt politicians held power on their home planet of Cybertron. While the older "gladiator" history was retained, Megatron's rise to power was now given rationale as the future tyrant initially saw himself to be a freedom fighter trying to overthrow oppression. Unfortunately for pretty much everyone, the would-be oppression-ender ended up becoming the oppressor himself.
In the original days of Transformers, Megatron turned into a realistic Walther P-38 pistol (with attachments for long-range capacity). I haven't been comfortable with this alternate mode being in my home for some time now, and thus I have no pictures of my own to share (they're not hard to find if you really want to). I'm clearly not the only person to feel this way, as Hasbro themselves have had issues with releasing Megatron in this form in recent years (even the tiny Legends scale toy, released in 2010, ended up with a bright orange safety tip). As a result, Megatron has had a fairly wide variety of alternate modes over the years, although variations on the tank form seen here are the most common. This enables Megatron to retain the impression of having tremendous firepower without turning into something that lawmakers fear might possibly do real harm to an actual child someday.