Why not be awful?

I was going over the Sequences on metaethics, and it was leaving a bad taste in my mouth. The examples are all about killing or saving children (both of which are far outside my personal experience). The assumption is that the participants in a discussion about metaethics are, in fact, moral in the normal sense of the word. That they’re talking about justifications behind beliefs they actually act on, like not killing babies. That, when the philosophical discussion is over, they will go back to being basically good people, and so part of the purpose of the philosophical discussion is to explain to them why they shouldn’t stress out too much. If there were no “morality,” you still wouldn’t kill babies, Eliezer presumes. Philosophy is just so much verbal dressing on something basically secure.

But my situation is a little different. From time to time, like Pierre, I don’t care. I get emotionally nihilistic. I find myself doing things that are morally awful in the conventional meaning of the word: procrastinating, sneaking other people’s food out of the communal fridge, being casually unkind and unhelpful, breaking promises. I don’t doubt that these are awful things to do. I figure any moral theory worth its salt will condemn them—except the moral theory “I don’t care,” which sometimes seems strangely compelling. In an “I don’t care” mood, I generally don’t care about the truth or falsehood of factual claims either. What does it matter? Penguins are green and they are a deadly menace to human society.

What I want to know is: what goes through people’s heads when they’re motivated not to be awful? What could you tell someone as a reason not to be awful? If you are, in fact, not awful, why aren’t you awful?

Edit: the kind of why I mean is not a justification (Humans have natural rights to life, liberty, and the pursuit of happiness) or an explanation (Humans care about the things evolution leads them to care about.) I’m talking about an internal heuristic or a gesture at an intuition. What do you think, or feel, when you care about things? What would you tell someone who claims “I just don’t care” if you wanted to get her to care? What would you tell yourself, in your nihilistic moments?