Here is a discussion I had with a nihilist. Perhaps it will help.
Nihilist: "Evil" LOL You're presupposing that there is even absolute morality in the first place. Haven't you ever heard that you can't get an ought from an is? Morality is just nonsense dude.
(Me)Moral Realist: So are you saying because the is/ought problem is true I ought to not believe in this thing called morality?
Nihilist: Uhhh.... (hesitates).... no. I'm just saying you can't get an ought from an is and that you are wrong.
Moral Realist: Well if there is no reason to change one's behavior upon the perception of a new fact then what the heck is the point of this dicussion? Isn't is self-contradictory to say someone is "wrong" when you don't even believe in morality in the first place?
Nihilist: The point is to uncover the truth. By "wrong" I meant to mean what is false.
Moral Realist: Why is the truth important?
Nihilist: Because the truth has utility to me and I would assume the truth has utility to you as well.
Moral Realist: I know full well it has utility to you but why? It is illogical to have preferences for no reason.
Nihilist: Because the truth will help me survive and help me get what I want.
Moral Realist: Why is survival a value? What exactly do you want and why? Why should you get what you want?
Nihilist: I just want the fullest happiest life possible dude. What's so bad about that?
Moral Realist: Why?
Nihilist: Pfff, I don't know. I just want it.
Moral Realist: Since you can't make sense of your preference for the fullest possible life let me offer a suggestion. The very point of a discussion is to uncover the truth but the truth is not just a subjective value held by both parties. It is a universal preference. Truth is universally preferable to falsehood. Why then is survival a value? Beause survival is necessary for one to accumulate the truth. Why then should one seek pleasure? Because rationality is the prime virtue in accordance with the universal preferability of truth. Rationality is also a very taxing behavior an organism partakes in and the organism must recouperate in order to maintain rationality. Why should one try to be civil? Because one can benefit from the rationality of other people. Why are universal morals better than relative ones? Because conflicts of interests are caused by conflicting moral rules. This is a much more complete philosophy than a mere assertion of desire.
Nihilist: You're so wrong dude. I disagree with everything you just said. I disagree that the truth is universally preferable to falsehood. I'm not talking with you under the premise that truth is universally preferable. I'm talking to you under the premise that the truth is at least preferable to me and you.
Moral Realist: The problem is you can't make sense of your preference for truth. It is illogical to have have preferences without reasons.
Nihilist: I don't see the reason why I have to "make sense" of my desires in the first place.
Moral Realist: So you're a sociopath then?
Nihilist: Uhhh...no. I'm not a criminal.
Moral Realist: Well just because you you're not a criminal doesn't mean you aren't a sociopath. A sociopath is defined as someone who doesn't feel they have to justify their own preferences. Whether they actaully hurt people is irrelevent.
Nihilist: Even if I was a sociopath so what? Why does it matter?
Moral Realist: Because if you can't justify your own preferences then you will inevitably become a victim of those that are stronger, smarter, or fitter than you. Nature is governed by natural selection.
Nihilist: But even wolves cooperate with each other. Just because the earth is governed by natural selection doesn't mean those of the same species eat each other.
Moral Realist: That is true but wolves cooperate with each other because of instinct. Instict gives them an automatical knowledge of right and wrong. Mankind does not have an automatic knowledge of right and wrong. Right and Wrong are determined by man's reason. Men are capable of cannabalizing each other to a far higher degree than other animals because they do not act primarily on instinct. Since right and wrong must be determined by reason what reason is there for anyone to respect your preferences?
Nihilist: I don't think man is as non-instinctual as you claim, but I guess it would be best if not too many people think the way I do. Nihilism is okay as long as not too many people believe in it.
Moral Realist: Which is precisely the definition of what it means to be wrong. Other people shouldn't think the way I do!
Nihilist (Laughs, shakes his head and leaves)