Firstly, you assume that by moral nihilism I mean that I do not believe morality, as a social construct, exists or is valid.

That's not true at all, and I will happily stand up and help to enforce the morality of ensuring nobody does, of doing what's best for your nation, and for those in other nations to do what's best for the world.

My point is exactly the same point you make in your post. Morality is arbitrary, it is defined by our feelings and attitudes about the world. Morality is also something that is larger than the individual. Yes, your example of killing is an accurate one, but that ignores that we do work as part of a larger group, and that group can set rules that allow us to kill one another, at the ignoring of our "basic" moral instinct, while still acting in a way that produces long-term benefit and success.

Society allows us to obfuscate moral choices, to make choices we would call immoral, but whose benefits are well worth the costs. We put the morale of killing animals that we clearly know to suffer behind the minds and actions of a select number of butchers, who cleans and sterilize the process so much that we just buy a package from a grocery store.

We put the process of enslaving, or abusing people for their ability to perform labor, and the pain caused by that behind the minds of other societies, or behind the idealization of markets. We will do our best to ignore empathy, to ignore the morality we assume is better-by-default, when it best suits us.

People taught the "empathetic" view of morality from childhood look at these things are are shocked, horrified, at how "we" can cause so much destruction and abuse, but they fail to realize that's the nature of society. It's a decision making engine, from my point of view, and the decisions it makes are correct at the end of the day, although there are times it goes wildly off course, and we often think of those times when we think of "evil".

My argument is that the reason we live in a society that values us so greatly, that feeds us so well, that pampers and cares for so many within it is largely thanks to the fact that doing so benefits society at the end of the day. If it didn't, if we needed to be starved, worked to death, so that society could continue, our social structures and beliefs would shift until those things became commonplace, accepted, and rationalized.

It's like I said, we lived thousands of years, and practiced slavery all across the world. It doesn't end until right around the era where mechanized labor becomes popular, and where factories replaced the uneducated laborer. We didn't suddenly realize slavery is immoral, slavery became immoral, and society changed as a result.

We live in an era, despite our best intentions, where forms of slavery are still moral, and will continue to be practiced, even if your kindergarten class taught you that right and wrong are determined by nothing but feelings and the golden rule.

Additionally, we can discuss individual morality all we like, but individual morality is meaningless. We always make decisions we feel internally are correct, so what's the point of ever discussing what I feel is right? What I feel is right doesn't change anything, it's what I do to effect the actions of people around me, and how I pressure and how the system I live in pressures me that matters, that pressure is morality, and the actions that system attempts to create are the "moral" ones.

I'll guarantee you that when we stop torturing animals it'll be thanks to an innovation in lab-grown meat, or it will be due to global warming causing our consumption of meat causing the practice to be unsustainable, not because humanity suddenly realized it was wrong.

The same will occur when we stop abusing the third world for labor, It won't be thanks to a sudden moral realization, but because the world changed, and helping the third world is more important than using them for acquiring cheap plastic toys.

Should we stop? I as an individual may believe that it is wrong, but at the end of the day I'm still buying a chicken sandwich and shopping at walmart.

Society is bigger than us, smarter than us, and sets us up from birth to act the way we do. There is no hope, no practicality, in fighting it, unless you believe society itself has recently seen a change in the forces against it, such as fighting for abolition leading up to the civil war. If the forces that effect society aren't changing then you will be a single man against a hundred thousand, and the most you will be remembered for is, at most, being ahead of your time.