Exploring the real-life consequences of The Purge would actually be more interesting than the movie probably is.
While it's "legal" to do all sorts of stuff for 24 hours (EDIT: 12 hours in the movie), it would also be legal for the police to summarily execute you for doing any crimes. Sure, you could shoot your boss and not be "charged" but you'd immediately be gunned down by someone else. So, most people would spend those 24 hours bunkered down in their houses armed to the teeth ready to immediately execute any tresspassers, while the security forces would be roaming the streets ready to immediately terminate any looters or miscreants.
Also, a big plot hole is that committing a crime in the 12 hours is legal, but you'd have to prove that it was in the 12 hours. For example, if you murdered someone with a few minutes to go so that you couldn't be counter-killed with impunity, you'd have to have proof the killing occurred in the 12 hour time-window, and you can bet that lawyers would make the most killing, trying court cases for years using forensics that they show prove the actual killing took place just after the cut-off point, not before it. If you just left a body without evidence then they could fudge the forensics and say it's outside the time-window, or at least not provably in the time-window. So you'd have to document your kills and get proof of the time of the kill.