The purge is about the government reducing the population, mostly the poor & disadvantaged. They came up with a scheme that would appeal to all the sickos like themselves! An annual time of the year when you could kill with no legal consequences.
I thought this was a great movie, full of action. A few loopholes, things you & I would not do, but on the whole worth watching. The scary part is that I can see this could easily happen here in Australia. We have a totally corrupt to the core government who is actually paying the Indonesian government to kill our WW2 Allies in West Papua. This is the genocide that the world in ignoring! So yes, I think our sick government is quite capable of turning on Australian citizens if it means gaining more power, control & wealth. Remember, it is now illegal in Australia to purchase, own or carry anything for use in self defence of for the defence of others!!! Think about that!