Green Room is a coldly efficient picture about punks, neo-Nazis, and violence. Jeremy Saulnier has cited Straw Dogs as an influence, and I suppose Green Room does call on some toxic masculine violence. But unlikely that repulsive film, Green Room is at least an equal opportunity revenge thriller. By the end of the movie, of the two remaining heroes, one is a woman, as if Dustin Hoffman in Straw Dogs couldn't succeed until he got a woman to join him in brutal violence. It helps that Saulnier doesn't take himself as seriously as Sam Peckinpah did.
The protagonists are a punk band, three guys and a woman, whose idea of a good song to cover is "Nazi Punks Fuck Off" (it is a good song, but perhaps not the best choice when the crowd is filled with Nazi punks). They are gradually joined by another woman against what turns out to be a vicious fascist cult. Before the movie ends, we've seen a variety of killings and near-killings, involving (list not inclusive) slicing up a guy with a box cutter, people getting eaten by attack dogs, a guy being shot in the face, and another guy at the receiving end of a machete. You've been warned.
There are some good casting choices. This was Anton Yelchin's last movie released before he died (there were a few more released after, including Star Trek Beyond). He does a good job in what might be called the Dustin Hoffman role ... his descent into violence is believable, and we see the impact it has on him. Alia Shawkat is one of the band members, and she can do no wrong in my book. Imogen Poots is the outsider to the band who joins their side in the battle, and she is quite the badass.
But the casting everyone talks about is Patrick Stewart as the head of the neo-Nazi cult. He didn't work for me ... I know we're supposed to think "Damn, Patrick Stewart is a bad guy!", but he underplayed his part and thus got lost among all the violence.
You should be able to tell by all of the above spoilers whether Green Room is for you. I liked it. Rather improbably ranked #686 on the They Shoot Pictures, Don't They list of the top 1000 films of the 21st century.