The point is that you can't say that McKinsey are incompetent and not worth the money, while at the same time saying they are so competent that they created the opiod epidemic. It makes you look silly.
Doing good economic work for bad people is a (gigantic) ethical flaw, but not a problem with the economic quality of the work and skillset.
Are consultants something like lawyers, in that they will do "good" work for ethical clients and "bad" work for unethical ones? Lawyers can point to the higher values of truth and justice. What are the higher values of consultants, to help them avoid "bad" work?
1. McKinseys advice is for the most part useless , often their advice is just politically motivated within the clients offices
2. They have zero morals, they will advice how to optimize the gas chambers (probably not too off the mark, they probably are advising something related to uighurs somewhere, maybe how apple can hide traces of genocide fueled chips from their product lineup) if they can make a couple mills.
3. The despots/murderers recruiting them are still going to go ahead with their addiction rings/gas chambers irrespective of what McKinsey tells them. The argument isn't that McKinsey made the opioid problem worse. Maybe they did but they probably will conveniently agree with us that they were not really consequential. Still doesn't mean everyone from McKinsey shouldn't be considered a dick though.
Doing good economic work for bad people is a (gigantic) ethical flaw, but not a problem with the economic quality of the work and skillset.