Anime / Manga - You might have killed millions of people, but at the very least, you made me happy

When you choose B.

This is what's known as the "Identifiable victim effect", where something abstract and vague like "killed a million people" doesn't instill as much empathy and shock in a person as something close to their proximity like "killed a single child living down the street." There are interesting readings on this particular phenomenon, mainly over on Wikipedia and under TV Tropes' "A Million is a Statistic" page.