To perform an act that is morally wrong is to offend against humanity, and all offenses against humanity are equally bad.
Principles
All offenses against humanity are equally bad.
Perform an act that → offend against is morally wrong humanity
Since murder is morally wrong, it's equally bad to have killed one person with a bomb as it is to have murdered a hundred people with that same bomb.
Goal
Let's find an answer either going from "X was morally wrong" to "X offended against humanity", or let's find an answer saying "this instance of morally wrong / offending humanity" is equally bad as .