One of the main themes is the moral struggle. In the Injustice Comic Book Story, characters are faced with difficult choices that test their moral compasses. Another theme is power and its corrupting influence. The story shows how those with great power can easily be swayed to use it in unjust ways. Also, the theme of friendship and betrayal is prominent as characters who were once friends turn against each other due to the events that unfold in the story.
Batman is definitely one of the most interesting. He's always been a complex character, but in Injustice, he's even more so as he tries to oppose Superman's tyranny. Another interesting one is Harley Quinn. Her transformation and her actions throughout the story make her a character full of surprises. And then there's Superman himself. His fall from grace and how he becomes a dictator - like figure is really fascinating.
The story is about a twisted version of the DC universe. Superman, after the Joker tricks him into killing Lois Lane and their unborn child, becomes a tyrant. He then starts to enforce his own brand of 'justice' which leads to a rift among the heroes. Batman opposes Superman's regime and gathers a group of heroes and villains to fight against him.