you got to remember that Injustice was a fighting game, its story is expected to be kind of wonky with characters coming up with contrived reasons to beat each other up in between 3-4 minute cutscenes. And thats fine as it comes with the territory. But expecting that story and all its beats and drama to translate to a movie format, I assume this is a prequel but still- is a bit of a leap. Not saying it can't work but its something to keep in mind.
Stuff like Superman being powerful enough to completely take over the world but there are pills that even the playing field so Harley Quinn can beat up Aquaman in the next level. That kind of wonky stuff.