They are kinda both. In the recent IDW comic runs, Decepticons originally wanted to overthrow the Functionists. The Functionists were the ruling class of Cybertron who enforced the ideology that a bot's alt mode (what they transform into) should determine their occupation. This lead to a stratified apartheid society where the intellectual class was better off than the manual workers' class.

Megatron was among other things a philosopher and political thinker but because of his alt mode he was forced to work in a mine. He secretly writes and disseminates theory. At one point he even says "religion is the engex of the people". So the Marx parallel is apparent.

The problem is that Megatron also wants to exterminate ALL organic life for reasons I didn't look into. This is one of the main points of contention with the Autobots. Later in the timeline he is portrayed as extremely sadistic and bloodthirsty. So he is clearly not a character that one should be sympathising with. Decepticons in general are ruthless, brutal and destructive.

So it looks like the idea behind them is a liberal's understanding of communism with a fascistic angle thrown in?

Weirdly enough later they give Megatron a redemption arc where he esentially turns remorseful and good which seems completely out of place.

This is an example of the theory that Megatron writes: https://tfwiki.net/mediawiki/images2/3/36/Towards_Peace.jpg