All Might's debut as a Hero deeply inspired Chizome. Since then, Chizome aimed to become a Hero like All Might. Chizome entered a private Hero course high school to train to become a Hero. However, Chizome became hopeless and despaired after seeing the school's "fundamental decay"; that the students there were becoming Heroes for the money and personal gain. Disillusioned by the reality of Heroics, Chizome dropped out during the summer of his freshman year.
After dropping out of high school, the teenaged Chizome preached an ideology which he called the "Revival of Heroics" through street oratory speeches and soapbox speeches. However, no one listened to him.
Over the following decade, Chizome realized that only action will create his ideal world and change Heroics for the better; Chizome researched and disciplined himself in the training of the killing arts through self-study; he did this in order to achieve his duty of "Revival of Heroics". During this decade, Chizome's parents died (it is unknown what led to their deaths but it is believed that Chizome was not involved).
After completing his training in the killing arts, Chizome took up the mantle of Hero Killer Stain and begun advocating the "Revival of Heroics". In this ideology of "Revival of Heroics", Chizome (now known as Stain) stated the guidelines of being a Hero; that Heroes must not desire rewards or compensation, people can only have the title of Hero when they commit the ultimate acts of self-sacrifice. The ideology stated that the "Heroes" of the modern world are fakes who pretend to be Heroes.
Stain then begun implementing his "Revival of Heroics" by killing Pro Heroes, deciding that he would not stop his purge of the Pro Heroes until the world realized the flaw in the Heroics system. Stain's end goal with implementing his ideology "Revival of Heroics" was to create a just world filled with true Heroes.
(Source: Boku no Hero Academia Wikia)
