Treehouse
(2014)

Killian (J. Michael Trautman) and Elizabeth (Dana Melanie) are horrified to discover that both their brothers were murdered by three brothers that lived deep in the woods. The brothers, lost without the guardianship of their deceased grandmother and  warped with a twisted Christian ideology, have begun kidnapping and killing kids in town.

Killian and Elizabeth manage to escape their clutches and Killian kills two of the brothers. In the process of flagging down a car, Killian accidentally causes a crash, killing the driver, who happens to be a police officer transporting evidence. Killian and Elizabeth look inside the trunk of the truck and find numerous assault weapons. They each take a rifle and prepare to head back into the forest to kill the third brother. They do this with the knowledge that the authorities might charge them with murder afterwards, desperate to get justice for their deceased siblings.

Thanks Spectre!