In the majority of superhero stories, the superhero wants to hide his or her power, but they also want to use their powers to save others. The superhero is constantly in danger of being exposed, whether they use their powers to fight crime or they keep to themselves as much as possible. At some point, the hero is confronted with a choice: expose themselves to save someone, or don't. Protagonists usually chose to save that someone, and maybe they don't actually get found out by the whole world, but they took that risk because "With great power comes great responsibility."
But what if they didn't? What if they were a coward, or the person who needs saving wasn't so dear to them? What if they weren't The Hero? Or maybe their exposure carries higher stakes and there is no "right" answer. So, they let the kid die. Or their husband. Or a bus full of refugees.
What happens then? How does that change the themes of the story? How does it change the character? Would you write an antihero? Is it even possible to avoid anti-hero vibes if your character knowingly lets someone die?
Sorry, bus, but we just put the (anti)hero's life above yours. Cue evil author laugh!