My comment can stand on its own but it was given in this case as a general-interest supplement to your own, not as an objection.
What you've pointed out is a good example of how frequently some individuals ignore the division of descriptive and prescriptive ethics. The sanctimonious person you've illustrated is little more than an egotist, though often times the same attitude is also symptomatic of moral realism, or moral reification, which again rests upon personal construal of "natural law" and/or belief arrived at via mystical revelation.