The snow during the closing scene is not real. They brought two trucks of salt and scattered it all over the street; the falling snow is CGI.
Several actors in this film, especially the ones portraying the relatives of the dead children, also appeared in Sympathy for Mr. Vengeance (2002) and/or Oldboy (2003), the previous parts of the Vengeance Trilogy.
Having wanted to make a film on a middle-aged woman's revenge, the director originally considered casting Ko Du-shim for the part of Geum-ja. However, he had to abandon his plan for a couple of reasons. He found that Ms. Ko was rather old for the character and was afraid that the movie would look quite similar to John Cassavetes' Gloria (1980).
(at around 40 mins) Geum-Ja quotes Mr. Baek about how there are "good and bad kidnappings," with the good ones being those where the child is safely returned after a ransom has been paid and this then actually brings a family closer together and makes them happy. This whitewashing reasoning is exactly what Ryu's anarchist girlfriend told him in Sympathy for Mr. Vengeance (2002) to convince him to kidnap a child. It therefore can be seen as coming full circle from part 1 of the trilogy to part 3.
The bakery that Geumja works in is called "Naruse," which is the name of the Japanese director Mikio Naruse.