The Capitol

The Capitol are the primary antagonists of The Hunger Games, being the leaders of the twelve districts of Panem but also a fascist totalitarian society that brutally represses the twelve districts and keeps them under their heel. To discourage rebellion (and also punish an earlier one) the Capitol dictates that the twelve districts every year must offer up one young man and woman between the ages of 12 and 17 to fight in a series of brutal televised battles for the Capitol's sick amusement.

Ultimately though, as the situation in the 12 districts becomes progressively worse to the point that it can no longer be ignored, people from the various districts begin to act up, leading to a schism between the districts that remain loyal to the Capitol for whatever reason, and those who choose to stand up to them. In their desperation to suppress a rebellion, the Capitol at one point even destroys District 12 to punish Katnis Everdeen for her "defiance".

Symbolism
Aside from the clear parallels that can be drawn to Ancient Rome (totalitarian and militaristic society with gladiator-esque games) the Capitol and it's actions are also a blatant attack on modern day culture (or at least modern day US culture), in particular the growing desentization fo the masses to real life violence, and how real life violence is almost becoming as much a source of entertainment as fictional violence. In a cruel twist therefore, any who reads the Hunger Games books are in essence, taking on the role of someone in the Capitol, as the reader too is watching children fight and kill each other for "entertainment".