If the Purge were real, wouldn't it only last one night?
If the Purge were real, it would start and not stop. It would continue until a lot of the world's population would be dead, and the world become an Apocalyptic Wasteland. I'm sure someone with a Joker-like attitude would take the opportunity to storm the Pentagon or hijack a Submarine and unleash Nuclear Armageddon upon the world... I know that's what I'd do is they started the Purge. No one deserves to live in a world where they think the Purge is a good thing.
Reading the Headscratchers on TVTropes really puts into perspective how many plot holes this movie series has.
http://tvtropes.org/pmwiki/pmwiki.php/Headscratchers/ThePurge
EDIT: Also, on a different note, what's stopping someone from murdering a person on the regular day, hiding the crime, and then if it's discovered, saying that you did it on Purge Night?