But that's the thing. I don't think he does anything other than because he think it looks cool. Actual darkness is a bad fit for the Justice League generally but if you do go for it, it has to have substance, it has to have a purpose. It can't just be grimdark for the sake of it. And for that, Snyder is definitely not your man. He's good at surface (if you really like sepia, at least) but he is an incredibly vacuous filmmaker with nothing of any substance to say. His Watchmen was case positive of this. He lifted whole sections of the comic and put them on screen but the underlying point of Watchmen? The substance? That went straight over his head.
What you're talking about is basically the opening of Final Crisis but Zack Snyder is no Grant Morrison. Worst of all, he clearly thinks he's Alan Moore but he's closer, in fact, to Rob Liefeld.