My problem with it is that the dark, brooding edge is so very much at odds with the super heroes in the Justice League. Both Wonder Woman and Superman are aspirational superheroes, they are superior to normal human beings in every way and yet they protect us because they feel they have a moral obligation to do so. The point of both of them as superheroes in a meta sense is to inspire hope and confidence, making the reader feel that the strong will do the right thing and be decent people and will always be there to defend what's right (any analogy here to the USA as the world police in a post-WW2 world being entirely intentional in the comics). Tonally, that's completely at odds with what Snyder loves to do, which is show flawed, broken people get by by the skin of their teeth by making huge sacrifices and struggling to get past their flaws.
Snyder's tone and theme are all wrong for the superheroes he's writing and directing. I don't think it is a coincidence that Wonder Woman is considered the best of the bunch, because it understands that Diana can be in bad situations and witness a lot of suffering but that the core of the character is that she will try to fix it and inspire hope in everyone around her. If that's the core of the character (like with WW and Supes) you damn well better make sure the tone of your movie matches, because if you don't, if you make Man of Steel a brooding piece about a reluctant, insecure hero, then you're bound to mess it up. You won't be making a mature take on a beloved character, you'll be butchering the themes of the character with every scene.