Gygax divided "morality" (Good-Evil) and "Ethics" (Law-Chaos). This is muddy, since the words mean the same thing, one is Latin (Mos-Mores) the other is Greek (Ethos). Both mean the set of habits and behaviors of a given person.
The meanings given by the web-page that Nivenus cites have no real basis either in the classical languages from which the words are derived, nor any standard English dictionary. See eg
this dictionary website which gives as its opening definition of "ethics" the completely orthodox "system of moral principles".
The website Nivenus refers to contrasts social norms and personal convictions. Not only is this not a particularly canonical use of ethics vs morality, it is not really relevant to D&D either, which has no theory either of social life or of individual conscience. For instance, if a devil is, in its nature, LE - as per the quotes from the 5e rulebooks upthread - than what does it mean to talk about socialisation of it into certain norms, or to talk about its "personal compass of right and wrong"? Furthermore, it is completley normal to use the word "morality" to describe a system of social norms - eg scholars will talk without any awkwardness or hint of contradiction of "the morality of a society", meaning the set of evaluative commitments that are typical of the members of that society, and/or that are inherent in that society's modes of social organisation.
In contemporary Engish-language philosophy, "morality" is generally used to refer to the body of norms that set out our other-regarding duties. "Ethics" is often used as a synonym, but when it is not so used, it is used to encompass morality together with any other norms (eg self-regarding duties) with which compliance is necessary in order to live a good life.
It is possible to shoe-horn D&D alignment categories into this standard philosophic useage - eg "lawful" means "honouring self-regarding duties", in the way that a monk does but a disssolute bard doesn't, and "good" means "honouring other-regarding duties". The difficulty is that, in real life, the monk and the bard (assuming the bard is not dissolute simply out of weakness of will) disagree on the existence and content of self-regarding duties, and hence over whether or not "lawfulness" is a genuine evaluative category. But in 9-point alignment D&D this intellectual manoevure is not available, because both the framework of alignment and the corresponding framework of the outer planes tell us that there really is such a thing as lawfulness. So the bard has to admit both that self-regarding duties exist, while maintaining that they do not deserve to be honoured. Which makes no sense.
Law and Good ARE DIFFERENT. Otherwise, you'd have to say that Asmodeus and his Devils were shining examples of virtue... to say nothing of real-world baddies like the Nazis, who had a rigid, orderly society.
Why would I have to say that Asmodeus is a shining example of virtue? There are any number of reasons for thinking that Asmodeus and his devils in fact lack honour and self-discipline - that their surface cultivation of obedience to external norms is subverted by an underlying selfishness and penchant for manipulation. And the proper analysis of National Socialist legal practices probably falls foul of board rules, but I refer you to Lon Fuller's well-known work on the "grudge informer" cases as well as Rundle's fairly recent article in the Toronto Law Review on "The Impossibility of an Exterminatory Legality".
And of course, if I take off my natural law hat and put on my existentialist hat, I can flip all the above around: self-abnegation to externally-imposed norms is bad faith, and hence unworthy of any human being. On this approach, Asmodeus is in some ways worse than a paladin - because Asmodeus exploits the bad faith of others in a cynical way, whereas the paladin is perhaps just naive (as per Weber, in "Science as a Vocation" - "the arms of the old churches are open wide" to those who cannot cope with the reality that the external world does not create or impose vaue). But in other ways the paladin is worse, because the bad faith goes all the way down, whereas at least Asmodeus recognises his own participation in structures of bad faith.
Law and Chaos HAVE NOTHING TO DO WITH MORALITY... let me repeat, being a good person has nothing to do with how many laws you follow, nor does breaking a law necessarily make you evil.
You can assert this stuff, but it doesn't make it true (putting to one side that few D&Ders define "lawful" as "following laws").
For instance, the authors of the American constitution clearly thought there was an intimate connection between law, justice and social welfare. That's why they went to such pains to constitute the country as a rule-of-law republic rather than an absolutist monarchy of the style then popular in Europe.
Or, to take a completely different example, orthodox Theravadan Buddhism thinks there is a clear connection between self-discipline and wellbeing. That's what meditation is for - it is crucial to the release of the individual from sufering.
Conversely - once again moving away from various "natural law" approaches - the anarchist who maintain that "law" is a morally meaningless category, and that every human action has to be evaluated indendently of its legality, are not taking the view that lawfulness has no bearing on wellbeing. They regard reverence for law as inimical to wellbeing. Or to take a pre-modern outlook that has some affinities to the modern anarchist, namely the "heroic morality" that one finds in (say) the Homeric epics and the Norse sagas, there is a clear view that part of being a decent, worthy peson is being capable of extravagant self-assertion (which, in D&D terms, would be "chaos").
My point is that there is no real-world moral framework, no real-world systems of moral evaluation, no system of real-world normative practices, that regards matters of discipline, honour, self-cultivation, self-assertion, self-restraint (which is the function of law in a model of republican government), social stability, social order, etc as orthongal to matters of human welfare and wellbeing. Whereas 9-point alignment is committed to this ostensible independence of the two things, such that human wellbeing can be fully realised both in a framework of law (eg the Seven Heavens) and in its absence (eg Olympus).
But if this is really true - if it is really possible to fully realise human wellbeing both in the Seven Heavens and in Olympus - then choosing to live a lawful life or a chaotic life is of no greater moral significance than choosing to where a white shirt or a cream shirt (ie purely a matter of personal taste and inclination). In which case there would be no warrant for lawfuls to regard chaotics as in any way flawed, nor vice versa.
Which goes back to [MENTION=5142]Aldarc[/MENTION]'s remarks about the emphasis, in AD&D, on good/evil as the main conflict. I think this is because most playes intuitively realise that within the 9-point framework the choice between law and chaos really is nothing more than a matter of personal inclination, and hence of no moral weight. The fact that dwarves have stiff necks that irritate the flighty elves (and vice versa) is no different from the fact that when you make an appointment in Germany people are more likely to turn up on time than if you make the appointment in Argentina. Or the fact that an American film is more likely to have a sentimental ending than a Russian film. These are difference of individual and collective character, but they don't have any moral significance.
If you want to give law vs chaos moral weight - as, say, Moorcock and 4e D&D do - then you have to connect them to more fundamental notions of order and dissolution. But at that point you simply can't pretend that there is nothing to choose between them as far as human well-being is concerned. You will have to take a stand as to whether human well-being is best served by order, by dissolution, or by neither (or perhaps both). This is what Moorcock does. This is what the PCs in a 4e game have to do.
But in Planescape doing this would require denying that (at least one of) the Seven Heavens or Olympus is
really a good place. Which would require departing from the alignment/planar framework as written.