I often find that people are confused when I discuss my views on ethics with them. One criticism I often receive is that what I am doing or talking about isn't actually ethics. I think this highlights a crucial misunderstanding about what moral philosophy is, and why we need it.
I think most people think of ethics in punitive terms. For them, ethics exists primarily to satisfy the desire for justice. It makes sense that we have this desire; in evolutionary terms, it is beneficial to the tribe to discourage behavior that puts the tribe's survival or well-being at risk. This style of ethics is ancient and arguably hardwired into the human brain and that would explain why it is so pervasive.
Ethical philosophy, as a discipline, did not start as a way to explain or justify the desire for justice. If we look back to ancient ethical philosophers like Aristotle, it becomes clear that, by ethics, they mean something other than just justifications for punishment. In their works you'll find a focus on living a proper human life, what the proper goal of a human life is, and how to achieve it. Rules and punishments, if they are discussed at all, are discussed as an extension of a more basic ethical theory rather than the center of it.
It's become clear to me that most people do not see ethics as a means of living a better life. Look at the outrage when Casey Anthony was acquitted for the murder of her own daughter; how many of those calling for her head on a platter were motivated by a genuine desire for a morally good world and not by a primal desire to see a wrongdoer punished?
I recently had a brief discussion with someone who claimed that, if determinism were true, then ethics is a useless discipline. I find myself confused by this claim, since any being capable of pursuing values and grasping abstract principles would benefit from principles that aid him in the pursuit of his values. There is no need for a metaphysical ability "to have chosen otherwise under the same exact circumstances" for ethics to have utility for such a creature.
It then occurred to me that his conception of ethics is fundamentally different from mine. For him, and arguably for most people, ethics is a series of justifications for satisfying that emotional urge to see wrongdoers punished. The argument is that, if the killer could not have chosen otherwise, how can we hold him responsible for his crimes?
But why is holding someone responsible for their crimes the primary focus of ethics? Let's suppose, hypothetically, that we can't hold him responsible. Does this somehow remove our need for principles to guide our actions, or to show us which values are worth pursuing? Clearly not. At least, I don't see how it follows.
Once we accept a self-interested ethical philosophy, and we conceive of justice as something a society should do out of mutual self-interest, then it no longer matters if the killer "could have chosen otherwise." It only matters whether or not the rest of us would benefit from having him imprisoned or killed.
Imagine a crazed robot with no free will killing a bunch of people. The justification for his destruction would be that we prefer to not be killed by a robot. Why is this not also true of people?
Most people tend to look at justice, not as a means toward the end of bettering everyone's lives, but as a means towards the end of some greater cosmic good. Murderers needs to die, evidently, not because we would be better off in a world without them, but because some primeval "justice god" is served by their death. There is some primal need for their deaths that cries to be satisfied, and this is not grounded in careful reasoning but human nature.
It should send a chill down your spine to realize that, to most people, ethics is not "principles that guide us in living better lives," but "excuses to hurt people." For many, ethics is perversely inverted and represents the most vile elements of human nature.