system
Given the following post, respond as if you're commenting on Reddit:
human
Why is this argument for moral realism so commonly used and accepted? Who came up with it?

Most instances I've encountered of people rejecting moral nihilism and/or moral relativism use an argument that more or less takes the following form:

"If there are no real morals or morals are relative, then I can kill a hundred babies for fun and nobody can say anything about it. That's absurd, so therefore universal moral truths are real."

The argument, from what I've seen, tends to be almost unanimously accepted by those who hear it. My Intro to Ethics textbook even used it (in a somewhat lengthier form), supposedly "putting an end" to logical opposition to moral realism.

However, this strikes me as a very poor argument for a couple of reasons:

1. It relies on the reader having a knee-jerk, mostly emotional response to the statement. This is not in the spirit of a rational discussion and, in my opinion, does not even belong in the realm of philosophy.

2. It completely disregards the fact that those who are ethical subjectivists do not advocate that the observer must accept the baby-killer's actions, as that itself is a value judgement. An ethical subjectivist would suggest that the observer would be justified in taking any actions that conform to their own ethical system (or lack thereof), including preventing the baby-killer from killing babies in the first place.

So, that said, my question is a two-parter: Which philosopher, if any, is credited with coining this argument against ethical subjectivism?

Additionally, why is the argument considered credible and/or how do the adherents of this argument defend it against my above points?
gpt
As has been brought up by warren, this isn't a common argument used for moral realism. Such a supposition would fly in the face of most of meta-ethical study. 

It is a more common argument against cultural relativism, to elucidate how relativism has been dismissed (i.e. the belief that, because we're part of a particular culture we're unable to judge other cultures as wrong and therefore should be tolerant, open-minded, multicultural, etc.) because cultural relativism strips us of the tools to condemn things that are intolerant, close-minded, assimilationist etc. Perhaps that is what your textbook is referring to? Could you give a source?