Prime showed that Megatron started as a freedom fighter who got drunk off the power of becoming the new tyrant after he and Optimus overthrew the unjust rulers of Cybertron. It was the whole "I didn't actually want justice, I just wanted a turn to be the dictator" mindset. As such, he maintained that the Decepticons were still the underdogs even as they were terrorizing the innocent. A very liberal motivation for a villain, honestly.

His eventual fate at the end of Prime was really cool too. I'd elaborate, but I can't find any way to spoiler tag here....