Shirou Emiya wanted to help people, and now he was in a world that desperately needed his help. Even if he didn't know how he got there, or when even since apparently he's been there for a long time. But let's try not to get too hung up on that, there's people to save, diseases to somehow stop, and a girl he's choosing to try to save, even if everyone in the world says he shouldn't

[Dumb One Shot] When a hero dies, another rises. When that hero turns out to be terrible, one rises from beyond the grave to end everything.

Shirou Emiya thought that he lived in a normal world. Being stabbed and killed by a sludge wolf monster, only to be revived and turned into an armored superhero made him reevaluate that idea. At least now he can actually become a Hero of Justice. I mean it's not like he has to worry about his life anymore. After all, he's already a Ghost.