They're not opposites, they're orthogonal. Flowers for Algernon is a science fiction story. Science fiction stories are usually (always?) about how the human experience is affected by some hypothetical new technology. The new technology often has a fatal flaw or unexpected consequences. The stories are often tragic.

Captain America, on the other hand, is not science fiction. Captain America is a comic superhero and his stories are usually (always?) heroic adventures. Heroic adventures follow a predictable pattern beginning with a call to adventure and ending with the hero returning home having slain the dragon.