I voted to close it for a few different reasons.
As it stands, the question is asking whether there has ever been an instance where "a superhero" has killed themselves. This potentially encompasses tens of thousands of properties, maybe hundreds of thousands. Even the most cursory search revealed a half-dozen in the very first link
You've not defined what you mean by superhero. Are you only after comic book characters or would you also include film, radio, TV?
Before it got closed you had three equally good answers, all offering one example per answer. If it gets reopened, you could end up with potentially dozens of equally good answers, each offering a single example.