McKinsey bears some responsiblity but not all of it; nobody forced people to take those opiates, the addicts also bear some responsiblity. The idea in modern society that people have zero responsibility for their own actions is part of why we're in this mess to begin with, because people with that attitude aren't going to work hard to take responsibility for their own health and wellbeing. I'd bet at least half the people here complaining about McKinsey's behaviour are the same people who'd argue that people should just trust their doctor and do whatever the doctor suggests, and refuse to accept that often what's financially in the doctor's best interests is not in the best interests of the patient's health.
I am not defending what the firm did but we are talking about a study that took place in 2017, it has nothing to do with what happened before and certainly doesn't account for all deaths after 2017 either.