You can go dark with Superman. Not the character himself, but the things he faces. It's been done many times successfully. But when you forget Superman is supposed to strive to be better, strive to do better, than those around him, it tends to go to trout. And it's not enough to just pay lip service to it.

Go watch Superman vs. The Elite if you'd like a fairly dark Superman film that works well.

Don't get me wrong: I like Man of Steel, but with the washed out colours and too little of Superman getting to stand as an example for people rather than just reacting to villains, it wasn't a perfect Superman film.