The truth seeker objection against moral anti-realism

Russ Shafer-Landau states the following: 

". . . among those who have thought longest and hardest about ethics, the view that morality is all make-believe, or that all moral standards are correct only relative to individuals or societies, is deeply controversial. There are lots of problems with such views. Some of these problems may be devastating." (The Fundamentals of Ethics, 5th ed., pg 3)

I suggest that some of these problems are devastating.

One problem for anti-realism is what I call the "truth seeker objection".

1. If anti-realism is true, then a virtuous, intellectually virtuous, smart, well-informed, genuine truth seeker who limits his behavior to the facts could, in theory, commit a mass shooting and be perfectly within his epistemic rights in doing so, not expressing any kind of irrationality, poor reasoning, lapse of judgment, psychological flaw, or lack of knowledge of any kind. 
2. There's no way that a virtuous, intellectually virtuous, smart, well-informed, fully rational, genuine truth seeker who limits his behavior to the facts could, in theory, commit a mass shooting and be perfectly within his epistemic rights in doing so, not expressing any kind of irrationality, poor reasoning, lapse of judgment, psychological flaw, or lack of knowledge of any kind.
3. Therefore, there's no way anti-realism is true. (modus tollens)

Put another way, if God came to Earth and transformed into a swan and raped a woman, or, to give an even more extreme example, if God were to send everyone to hell, this would not reflect poorly on God's character in any real way if anti-realism is true. God, knowing all facts, would see that there are no moral facts and could thus, within his perfect rationality, engage in the worst atrocities we can think of and there would be no fact of the matter that would render God in any way disconnected from reality or acting contrary to reality.

This is, like Michael Huemer would say, insane. Nothing could be more obvious in all of philosophy than the fact that people who engage in mass shootings are in some sense disconnected from reality or are irrational in some way.

However, Russ Shafer-Landau also says:

"There is no quick, knockdown argument that will demolish ethical objectivism. Nor is there any short and sweet proof of its truth. . . . Those who think that ethical objectivism is obviously false, or just as obviously correct, have simply gotten it wrong. Matters here, as elsewhere in ethics, are too challenging to admit of pat and easy solutions." (364)

Part of me thinks morality is obviously, and sometimes I even feel, certainly, objective. So what could it take to convince me that morality is not objective?

One avenue is through etiquette. I think relativity or error theory is true with respect to certain beliefs people have that are driven by social pressures (a good example is beliefs that have to do with gendered roles, gendered shame, social failures of meeting false or intersubjective standards of masculinity/femininity, and things like that). If someone can show that there is no relevant difference between etiquette beliefs and other key moral beliefs, then that would defeat my moral realism. The problem there is that I will argue that there are relevant differences between mere etiquette beliefs and more robust moral beliefs ("You ought not commit a mass shooting" is relevantly different from "You ought not wear a dress as a man", it seems to me).

Another avenue is more fundamental. Some people believe that logic is purely conventional and reflective of human psychology. If philosophy is grounded in a priori reasoning, and if a priori reasoning is grounded in human psychology, then philosophy is grounded in human psychology. If you can show that a certain fundamental skepticism is justified on the basis that all human knowledge is hopelessly constrained by limits of human intelligence and psychology, and that not a single thing we take to know in science or elsewhere is true in any meaningful sense outside of our human idea of truth, then you will have destroyed all sense of fact, and with it moral facts. So extreme skepticism -> moral skepticism is one option -- a nuclear option -- for establishing anti-realism. But if you open up the door to critical thinking and a priori knowledge, then you open up the door to critical thinking as applied to ethical contexts.

A third avenue is to deflate moral realism. This would be to admit that moral realism is true and we have moral knowledge of some very basic moral facts, but once moral dilemmas get into any real complicated territory (which is basically all moral dilemmas we face in real life), the fact that reasonable people can disagree shows that there's no accessible fact of the matter (or really, the fact becomes that it's rational to either phi or not-phi in that case). If so many moral dilemmas end in a "draw" then it's hard to see how morality is objective in any really useful way.

My response to this is that given the complexity of life, science, and discovering truth in general, we shouldn't be remotely surprised that morally complex situations demand serious time and effort to come to a well-reasoned and confident position. For a person to come to a well-informed, well-reasoned, confident position on any advanced topic in philosophy takes serious time and effort.

The problem is that when moral dilemmas arrive, you don't have that kind of time to resolve the issue at hand, unless you hire a bunch of philosophers to give you their analysis, but even then you risk getting pulled in different directions. Still, getting their input would help you come to your own resolution, and it's nice to know there are smart folks who agree with you and would, in theory, be willing to testify on your behalf as an expert witness. (Has a philosopher ever done that?)

At the very least, if you gave philosophers your initial thoughts, they could provide some invaluable constructive criticism of your reasoning and help steer you into a better plan of action, or bring out points in your defense, increasing your confidence.

Just as it's silly to expect for a single person to solve any sufficiently complex problem by themselves when it comes to any typical area of expertise, it's silly to expect for a single person to come to a confident answer to any sufficiently complex moral problem without significant help from many other thinkers.

They say there's building something cheap, fast, and well, and you can only pick two -- and I think that principle applies to philosophy in a way.

Another way I'd respond is that it's powerful enough to have moral facts over the basic or "trivial" cases, because all too often humans fail to act according to even the most basic and obvious of moral facts.