CONTAINS SPOILERS AND IDIOTIC STUPIDITY
The trouble with this film, which is otherwise a perfectly decent home invasion thriller, is that it's saddled with an idiotically stupid gimmick that it can't get over. It can't convince you that it's in any way a good idea, and it can't convince you that it could ever happen. If they'd ditched it early on, on the grounds that it's idiotically stupid, the resultant movie would have most likely been okay: nothing groundbreaking, but nothing to be ashamed of either. But they elected to keep the idiotically stupid gimmick, and so rather than getting excited at the dramatic possibilities they think (wrongly) are being explored, you just mutter "yes, but that can't happen" repeatedly at the screen.
The idiotically stupid idea at the heart of The Purge is that for twelve hours a year (7pm to 7am on March 21st), crime is legal. This is supposedly so that the populace of Future America can purge themselves of their anger, frustration and grievances without fear of reprisals and be better citizens the rest of the time. Ethan Hawke's top-of-the-range home security system (which he can afford because he manufactures them) is already activated when his son lets in the fleeing target of a bunch of homicidal sociopaths: they vow to break in and take their revenge....
Bollocks. Let's ignore the fact that Hawke's home protection system consists of nothing more than some metal shutters which are easily removed, without a second line of defence or even a panic room. Let's also ignore the filmmakers' overuse of the ancient gimmick of villains being shot on the back by characters you didn't know were there. The Purge, this cathartic primal scream of legitimised slaughter, is an idiotically stupid idea as it suggests that there's be nothing to stop you setting about a hospital with a flamethrower so long as you did it in between those set times. (Significantly, sex crimes are never mentioned.) There's just no way that's ever going to be voted through even in a future America run exclusively by whackjobs.
Oh, sure there's a subtext to it. The primary aim is actually to cull the homeless, the disadvantaged and vulnerable underclass: it's a necessary economic sacrifice made by the peasantry which also helps the rich white folks feel better about themselves as they stay indoors behind their expensive barricades. (For goodness' sake don't show this film to Iain Duncan Smith!) Or maybe, like Last House On The Left (either version), Death Weekend, the first Death Wish and so on, it's about how rich suburban liberals have to find the monster within themselves to fight off the monsters outside, and they don't like it when they have to get blood on their hands. Whatever. As a home invasion movie The Purge is an okay exploitationer with some crunchy violence, but saddled with the idiotically stupid premise of legal homicide it simply doesn't work.