Well - in the series' defense, there have been multiple times when Superman could have stopped/killed Batman - and vice versa - but in each case, the would-be killer stopped because they remember the dynamic they used to have, and how good it was for the world. Each of them wants things to go back to the way it used to be, but Superman won't compromise on his regime ideas. So they are at a moralistic standstill.
The reason for that is the series is connected to the video game, where Batman and Superman are alive. The only characters who can be killed are the ones who don't appear in the game.
If the series displayed Batman constantly on the run, never getting into a situation with Superman, cool. However, when he's in hand-to-hand combat with Superman, and Superman doesn't kill him, or Superman just lets him go when he has him dead to rights, it's pretty hard to swallow.
That being said, Injustice does one thing that most comics have forgotten how to do: tell a story in serial form. I keep coming back (out of review responsibilities, primarily) because I want to see what will occur next. It beats every DC book I'm reading in that regard.