Before Injustice even breaks the 15-minute mark, the Joker has killed Jimmy Olsen and Flash and nuked Metropolis to Kingdom Come, and Superman has inadvertently killed Lois Lane and his and her unborn son, as well as literally punched a hole through the Joker – this time very advertently. I doubt anyone would really miss Jimmy and Flash, and I'm not crazy about Lois either – but still, way to come out guns blazing right out of the gate.
In addition to wearing a weird-looking cape, Superman grows a five-o'clock shadow. Since this facial hair only appears in one brief scene, I must conclude that it's there specifically and exclusively to signal Superman's newly loosened morality.
Superman decides that he "won’t be held back by ideals that don’t protect the innocent," and he and Wonder Woman essentially hijack the planet and, among other things, bully Israel and Palestina into signing a treaty ("Peace by punching," Green Arrow calls it).
This causes a schism in the Justice League, with Batman, Catwoman, Plastic Man, Nightwing, Green Arrow, and Harley Quinn opposing Superman, Wonder Woman, Cyborg, Robin, and Ra's al Ghul, with whom Superman has allied himself to further solidify his heel turn.
Injustice is silly and brutal at the same time, and a lot of fun to watch as well as listen to thanks to its traditional, hand-drawn animation and clever script; cleverness that comes across not only in the droll dialogue but also in a delicious climactic twist that makes it clear that the only thing Superman has to fear is Superman itself (n the process explaining the weird cape).
Like All-Star Superman, Injustice accomplishes the rare feat of making the Man of Steel interesting. He remains an omnipotent, immortal god, but the film actually acknowledges his goodhood ("your will be done," as Wonder Woman aptly puts it).
By compromising his typically seamless moral fiber, the movie makes Kal-El more unpredictable and, in turn, less boring than usual (Injustice's take-no-prisoners approach loses quite a bit of its impact when we consider that "There are infinite Earths" with infinite Jimmy Olsens, so that what happens in one of them makes little or no difference; this bit of Fridge Logic, however, should not affect your enjoyment of Injustice while the movie is playing).