The Purge: Anarchy

One night per year, the government sanctions a 12-hour period in which citizens can commit any crime they wish -- including murder -- without fear of punishment or imprisonment. Leo, a sergeant who lost his son, plans a vigilante mission of revenge during the mayhem. However, instead of a death-dealing avenger, he becomes the unexpected protector of four innocent strangers who desperately need his help if they are to survive the night.

This is a current release, so it is not free to download. The legal options are renting, buying or streaming it on a service you already pay for. Every link goes to the store that holds the rights.