It's a graphic novel set in the DC universe. It likely involves the superheroes and villains of DC in a story filled with action, drama, and moral dilemmas. Maybe it shows an alternate version of the DC world where justice is perverted in some way.
In general, the Injustice comics are considered canon to the game. They often provide additional background and depth to the game's universe, filling in plot holes and enriching the characters' arcs.
It depends. In some main comics, injustice serves as a driving force for the story and character arcs. But not all comics focus on it as a central theme. It might just be a subplot or a background element.