> I and
> others have argued that moral judgments cannot be used as arguments, since
> there is no way to ground ethical propositions.
Absolutely. I don't think you & I are contesting anything of consequence; if anything, we're discussing different problems.
Your example ('Killing an innocent person is wrong') is on its face a universal proposition, like 'all men are mortal'. So of course one wonders about its truth value, and considering it in that light, one promptly encounters all the problems you delineate. There's no empirical basis for moral judgements -- not like the law of gravity, say -- and there's no a priori basis either (like the Pythagorean theorem). So groundless, yes, as groundless as you please.
The kind of moral judgements that seem more interesting don't take the form of universal propositions. Rather, they arise from the daily quandaries people face as they try to do the right thing. Seems to me that most people have a desire to do the right thing, as they have testicles or ovaries or a Muttersprache; that it's part of the usual human endowment. Of course the right thing isn't the same at every time and place. Which is why universalism is so stupid.
But at any given time and place there seems to be a lot of agreement about moral intuitions. Most of us would agree that we should spend more time with our kids, and also that we should drive carefully. So what's a tired frazzled commuter to do, coming home way late from the horrible office? He wants to kiss the twins goodnight before they're asleep. So does he drive slow enough to be reasonably sure he won't kill some inattentive or impaired pedestrian? Or does he step on it and figure, what they hell, they walk out in front of me, against the light? On their own head be it.
It's not a political question, or a philosophical one; it's a personal one.