The moral writings of Schopenhauer are some of the most compelling I have found despite their flaws. This is in part due to his reasoning but mostly due to his skill with language which is unsurpassed in the cataloging of philosophy which often is deemed dry and incomprehensible by even the most avid scholars and sharpest of minds. That being said, Schopenhauer commits numerous flaws in his ethics which deserves criticism.
The most glaring criticism one can make of Schopenhauer is he is inconsistent. He states that his project in ethics is only to give descriptive accounts of human morality, yet in his essay entitled Human Nature he claims that the belief that existence has no “moral significance” is the philosophy of the Anti-Christ. This is either him rebutting Psychological Egoism (a purely descriptive claim) or him saying that morality as conventionally understood, “ought-claims,” exist and can be known. It seems it is not the former because of the remark of the distinction between “…this significance and the world as it is” referring to what ought to be and the contrast between this truth and the truth of our world. He follows by a recommendation to not evaluate people based on their virtue (for in his view they have little) but instead by their capacity to suffer. Then empathy and kinship can flow from not only a common but universal fount. There are clear normative claims here as there are throughout the writings of Schopenhauer.
One could make the claim that since we must do something (even commit suicide) we have to have some theory or premise of “ought” to act upon even if we cannot ground it in certain knowledge. However, to make claims of what one believes and to make argument for what one knows through systems of logic are two entirely distinct things. If Schopenhauer were to claim, “I cannot know if I ought to be compassionate but this is what I believe is right and if you feel likewise then this is what I have found is most conducive to what I find to be right action,” then I would say he is consistent and reasonable. But to make the bold claim that ethics is not the study of ought-claims and then to have a philosophy riddled with moral accusations of people, strongly inferring that they ought to be kinder, stronger, smarter and so on, is a contradiction that is disappointing to find in a mind keen and knowledgeable beyond doubt.
In fact, one finds it hard to reconcile Pessimism, which above all else Schopenhauer is known for, with the view that he is only giving a descriptive account of things with no normative or “ought” claims. For to be a Pessimist is to take the view there is a stark and potentially irreconcilable chasm between the way the world is and the way it ought to be.
Though I have shown only one example of the contradiction of Schopenhauer I’d like to remain succinct and end this by examining a lack of understanding he has in other views. In Basis of Morality, Schopenhauer seems to misunderstand the Moral Skeptics which one would think he would be aligned with. The quote that Schopenhauer provides should not be interpreted as saying that all morality is nothing but human judgment and is therefore false. Though it can be read in this light, instead, they are making the claim that all judgments are known to be simply thus and the merit of said claim we do not know. This is different than the claim of the Stoics that what is good or ill is not things but instead our perceptions of them.
If Pyrrho and his followers are Moral Skeptics then they are not those who claimed that “no true morality exists.” Instead they claim that moral knowledge is impossible; for to claim knowledge of nothing being inherently “good” or “bad” would be to put forth a positive claim (roughly speaking Moral Nihilism). That is, either the Pyrrhonians words should be interpreted as being sympathetic to Moral Skepticism or they are not Moral Skeptics and Schopenhauer should not use them as citation for a view they do not hold or represent.
He states that what one does is determined by what one is but this in no way relates to either normativity (what ought to be) nor how humans judge actions or perceive normativity in descriptive accounts of ethics. He infers “Natural Ethics” to be grounded upon descriptive ethics because it is his focus to look at the motives of humans in a purely descriptive way. Schopenhauer as such acknowledges there is no way of discovering the nature of these descriptive ethics save the empirical.
From this premise, he then says we must search for that which is of genuine moral worth. But what is inherently of “moral worth” if we are not dealing with normativity but only the descriptive? By what measure do we consider an action of moral worth if not by the preferences of humans which Schopenhauer acknowledges has no normative weight? We have nothing to go off of but human whims (Schopenhauer believes compassion is moral and egoism base and immoral – a descriptive account that aligns with much of humanity but still does not make it correct) and the argument essentially becomes whether we are born feeling this way or feel this way through upbringing and experience. Either this, or it becomes a debate about the definition of “morality” that has no interest in normativity but instead in descriptive usage. If this is the case then I would concur with Schopenhauer that “morality” has something to do with compassion, or what is considered “good intentions” (but once again, what determines that such motives are good?) but this is akin to saying that “pleasure” has something to do with pleasant sensations. Making the claim regarding a word’s definition and language use tells us nothing new.
If we either remove or factor in a completely descriptive understanding into the normative language (e.g. “virtue,” “the good,” “justice,” “fairness” and so on) that he uses then there is much in his moral writings that stand scrutiny. For regardless of whether we ought to act one way or another Schopenhauer is one of the best writers on the moral intuitions of people. In this way he is much like the Greek writers that he mocks; unable to say with certainty why we ought to be happy, tranquil, virtuous or anything else but describes with clarity and understanding how we can come about these things in a purely descriptive way.
 To say that the world has only a physical and not a moral significance is the greatest and most pernicious of all errors, the fundamental blunder, the real perversity of mind and temper; and, at bottom, it is doubtless the tendency which faith personifies as Anti-Christ.
 Yet, however certain we may feel of the moral significance of life and the world, to explain and illustrate it, and to resolve the contradiction between this significance and the world as it is…
 The objection will perhaps be raise that Ethics is not concerned with what men actually do, but that it is the science which treats of what their conduct ought to be. Now this is exactly the position which I deny… The end which I place before Ethical Science is to point out all the varied moral lines of human conduct; to explain them; and to trace them to their ultimate source.
 There is nothing either good or bad by nature, but these things are decided by human judgment.
 Men are disturbed not by things, but by the view which they take of them.
 Consequently there remains no way of discovering the basis of Ethics except the empirical.