Because in a lot of ways Superman is the Superhero. He's one of the most recognizable, one of the most iconic, and his values of truth, justice, and hope have stuck with him for nearly a century.

Sure he's done a lot of brutal stuff here and there, and has had some weird comics. But when people think of Superman they tend to see him as that paragon of hope, that young man from Kansas who believes there's good in the world and it's worth fighting for even if it takes forever.

So taking that paragon, that icon, that ideal of hope and justice, then twisting it into something dark makes for an appealing tale for many.

Really there's all sorts of reasons why such stories are done. Sometimes it's a deconstruction, other times its cynical, other times it's to explore the terrifying idea of what if the best among us were monsters, and other times it's simply a means to make a dark storyline.

All the examples you gave are very different stories and I'd say that's the biggest reason these manner of stories are popular. There's just so many weird and fascinating tales you can tell in a world where Superman or a stand-in goes bad.