Injustice was the result of DC and NetherRealm Studios getting round a table and asking two questions; one was “how can we get the taste of Mortal Kombat vs DC out of audience’s mouths?” and the other was “how can we justify Superman fighting Batman?” The answer to both questions was make Superman evil, which they did in a way that, on paper, sounds kind of stupid.
In this reality the roulette wheel spun and it was the Joker’s turn to mind control Superman, and that he did.
In his confusion the big blue boy scout thought that his pregnant wife Lois was Doomsday (an obvious mistake, obviously) and accidentally killed her. As if that wasn't tough enough, a nuke had been rigged to blow when her heart stopped, taking out Metropolis. The utter devastation to Superman’s life made him snap, punch through the Joker, and start him on a path of being Earth's judge, jury and executioner.
As you might imagine, a super powered dictator would get up to some pretty evil stuff, and he did, including using his laser eyes to burn a hole through Shazam’s head, who remember, is just a kid in normal form, just for saying “hey, maybe we’re not really good guys anymore”. Way to prove him wrong, Supes.