Virtues and Vices: And Other Essays in Moral Philosophy Book

If there is a field in philosophy that I am not interested in it is Ethics. (Unless it is aesthetics.) But I thought I should read this one. I found her treatment of euthanasia and abortion quite okay. But as it gets more abstract I am at a loss. I cannot understand how one can seriously discuss the question whether it is possible that injustice is a good thing. Even if this is supposedly an ancient problem. A different question would be to take a look of what is considered just at different times or in different societies.
Or consider this statement: The primary criterion of goodness in a knife is its ability to cut well. (p. 135) Hm, really? Of course, it is, I am tempted to say. Isn't it totally obvious that what defines a good mother is to some degree a universal property and to some degree dependent on society? And rather boring it is as well.
Finally, I do not like to see Nietzsche treated, at least to some extent, apologetically.