It's one thing to create a weapon that can be used to kill O(100,000) people at once (though, it's not really "at once" if you do it by dropping N bombs consecutively). It's another thing to create a weapon that can only be used to kill O(100,000) people at once.

Or something. Of course, if inventing nukes is evidence humans aren't very moral, the fact that people chose to kill a hundred thousand people in Tokyo with conventional weapons is a different kind of evidence for humans being not very moral.