It's an amazing premise. We're talking about a world where 364 days out of the year, the few normal people who are naturally kind sit and watch weird assholes try to justify their shitty actions.

Even crazier: a world where non-purgers become vigilantes during non-purge days, taking vengeance on people who brag about their crimes. Nobody suspects them because they play it cool every purge day.

They become popular, get caught, and the movie ends in a peaceful revolution to stop the purge and end the series.