Ted Mills’s review published on Letterboxd :
A grimy, ultimately *too* claustrophobic Taxi Driver-esque portrayal of a suicidal hitman who gets more than he bargained for when hired to rescue a young girl from a child-sex ring. Everybody in this film is traumatized, and we soon figure out that the weapon-of-choice of "Joe" (Joaquin Phoenix, scarred and paunchy in body) is tied in to his own childhood. (This gets reworked near the end, suggesting that trauma has been passed down but in a different way.)
It's shocking, it will make you jump, but Ramsey clevely does a lot of the brutal violence off-screen, not that you notice this while watching. I would have liked to have spent a little longer with some of these characters--a lot happens wordlessly--but this is how Ramsey chose to do it.
Great score by Jonny Greenwood, too.