Despite his relatively brief presence in the story of My Hero Academia, the villain known as Hero Killer Stain has cast a long shadow over the subsequent chapters. He was defeated, but his message of heroes not living up to their title and being undeserving of the respect they're given resonated strongly with many people. It inspired other characters like Spinner and Himiko Toga to join the League of Villains, and it laid the groundwork for the current backlash against heroes following the Paranormal Liberation War. But who was Stain originally, and where did he come from?
The spinoff series My Hero Academia Vigilantes provides an answer, showing what Stain was up to years before he was finally taken down. Of course, he wasn't always known as Stain, nor was he always a villain. He actually got his start as a vigilante, intending to take out criminals and do the same kind of work that pro heroes do. While vigilantes are fairly rare, the formal processes to become a pro are quite difficult, so performing hero work illegally can be appealing to those who failed to enter a hero school. Stain began his career under the name Stendhal, a persona that he hoped might become a symbol in a similar manner to All Might.
Since the protagonists of Vigilantes are indeed vigilantes themselves, they initially regard him as a possible new ally. But where Pop Step, Knuckleduster, and the Crawler simply leave villains restrained for the police to deal with, Stendhal first reveals his darker side by slaughtering a group of Yakuza. However, one of the main problems in the Naruhata area had been "Instant Villains," people who were exposed, either intentionally or accidentally, to some quirk-boosting drugs and rampage out of control as a result. Since these people often don't know what they're doing, most would say that they aren't deserving of death, but Stendhal's fanaticism leads to a black and white view where all who commit crimes must be struck down.
Naturally, this means anyone who interferes with his mission is also immediately regarded as a villain and becomes a new target. This leads him into conflict with the other three vigilantes, nearly killing Koichi before Knuckleduster arrives, punching Stendhal's mask so hard it breaks his nose. Incensed that people who would call themselves heroes might stand in his way, he develops a new fixation: those he considers "false heroes," who are to him even worse than villains. In a brutal move of self-mutilation, he cuts off his broken nose and discards the Stendhal identity for good, and the transformation into Stain begins.
Stain's backstory as presented in Vigilantes provides an interesting counterpoint to the idea of the vigilante hero: pros must act in accord with the law, but a vigilante acts only according to his or her own judgment, and doles out punishment in accordance with those beliefs. While the efforts of Koichi and the others are shown in a positive light, it's really more a testament to their strong moral compasses rather than evidence that vigilantes are good for society. For Stendhal, all crimes are equal; however, in later chapters of My Hero Academia Vigilantes, many of these former Instant Villains come to the aid of both pros and vigilantes for good causes, providing essential assistance. Stain's rigid philosophy allows for no chance at redemption or reform, and if anything, his stint as a vigilante only confirms just how insane he truly was.
While he does turn to killing "false" heroes as a direct result of his fight with the vigilantes, it seems like that would have been the inevitable end regardless. When the world is full of shades of gray, a black and white view tends to result in misidentifying all but the purest as black. It's no surprise that Stain's ideology largely appealed to younger people, who didn't have the life experience to understand that. Stain's significance in the My Hero Academia story is almost a fluke—he just happened to be in the right place, at the right time, to become exalted by the very villains he originally wanted to stop.