Professor Eric Mack in the Fall 2002 The Journal of Ayn Rand Studies wrote perhaps the most comprehensive critique of Objectivst ethics. The next issue of JARS will contain a discussion of this article. I'd like to see how an Objectivist might respond to some of Mack's arguments.
He identifies several problematic arguments. I'll start with this one -- "rational parasitism." As Mack notes, there are entire species that live as parasites on others. With respect to human beings, certain people live quite well as parasites. Fidel Castro recently turned 80 and, while the Cubans are suffering under him, he hasn't done too poorly.