The process of Shiro becoming a red A
Since Cherie saved Corporal Lang in the fourth Holy Grail War, Shiro had the idea of becoming a "just partner". Red A is actually the future of Emiya Shirou. After the Holy Grail War, Red A did not choose to open a harem. But firmly adhere to the goal of becoming a "partner of justice", become a magician, go around the world to help people, and want to save most people.
When Shiro saved people in the power plant, the core of the power plant melted, but he wanted to save everyone in the power plant. At this time, inhibition appeared. He signed a contract with Shiro to let Shiro travel as a tool to protect all kinds of people in various world lines after his death, so he saved everyone. Shiro agreed.
But after Shiro became a hero, he kept repeating in the cycle of saving people and killing people. At this time, Red A realized that there was no happy ending for anyone. Later, he was falsely accused by his partners as the initiator of the war, was pushed to the guillotine, and Wei Gong died. He was also forced to the end by his own ideals.
Become a hero after death, that is, Red A, making endless atonement for his stupid goals and ideals.
follow-up action
In the process of saving the world, Red A denied the practice of sacrificing a few people, so he planned to return to the fifth Holy Grail War and personally destroy Emiya Shirou, who was still a student at that time. But because of the necklace, Red A was summoned by Tohsaka Rin to take part in the Holy Grail War, so there was the story of the fifth Holy Grail War.