Based on the book, "Death Sentence" - the direct sequel novel to "Death Wish," which itself was made into the movie, "Death Wish (1974)" starring Charles Bronson. The 1974 version spawned four sequels, none of which were influenced by the novel series. The author, Brian Garfield has disliked and does not care for each of the sequels due to how it departs from his original character, and theme in the books.
When Nick puts the gun to the head of one of Billy Darley's men, he asks the man where Billy is. The man says "Stygian Street." Stygian (1998) is the title of one of James Wan's student films. Stygian Street is also the name of the street of Jigsaw's first lair in Saw (2004), another Wan film. "Stygian" usually refers to the River Styx (near Hades, the underworld).
Despite the deviations from the novel, Brian Garfield was extremely happy with the film and praised it for understanding and making the central theme very clear.
When Bones Darley is opening up his gun safe, he quotes a line from The Crow by James O'Barr - "Fear is for the enemy. Fear, and bullets."
Leigh Whannell has been involved in nearly every one of James Wan's films. In this movie, he plays Spink, one of the members of Billy Darley's gang. Leigh had also co-wrote and starred in Saw (2004) and co-wrote Dead Silence (2007).