In traditional stories like epics, what do monsters represent? what do heroes represent?

Epic hero

The central figure in a long narrative who possess larger -than-life qualities such as bravery, loyalty, and heroism.

"decapitation of the government" is the idiom Jay Winik used in the second paragraph. The idiom's intended meaning is the severance of the top government officials from the remainder of civil society through assassination. I think the idiom was chosen to add a grave visual description of the sense of the moment.