Megatron was not always the viciously powerful and brutally direct leader of the Decepticons: he was once brothers-in-arms with Optimus Prime and a student of Sentinel Prime, appointed to be Cybertron's Protector and commander of its Defense Force. But Megatron resented his comrade, knowing he was a Prime and therefore Sentinel's favored son. This anger allowed The Fallen to ensnare him and led to the revival of the Decepticons. Even without The Fallen though, it's not hard to imagine Megatron would have erupted Cybertron into war to rid himself of Optimus.
Megatron tends to become so obsessed with the object of his desires that he can ignore "secondary" matters, glaring flaws in his plans. Otherwise, he likely wouldn't have chased after the AllSpark alone, and spent centuries entombed in the Arctic. Trying to talk Megatron out of blindly going for his goals is useless. He will risk his world, his troops, and even his own spark to achieve his goals; these things do not matter to Megatron. Like his master, Megatron feels a violent hatred towards humans and views them as only slightly above bacteria, a feeling exacerbated by spending decades as their prisoner.
ALTERNATE MODES
Megatron is the only Transformer thus far with more than 2 different modes...
In the first movie, he took the form of a Cybertronian jet.
In the second movie, upon being resurrected, he assumed the form of a flying Cybertronian tank.
In the third movie, Megatron receives another different alternate mode, that being a Mack 10-wheeler tanker truck (a demented version of Optimus Prime's alternate mode) with a tarp which may act as a cowl to partially hide Megatron's face due to extensive damage he received in the second film.
After his death in the third film, Megatron returns in the fourth movie, where his consciousness possesses Galvatron, a man-made Transformer created by KSI, and transforms into a 2014 Freightliner Argosy cab over truck.
