With opiate prohibition, Purdue had to convince doctors, and doctors had to convince patients, that the drugs they were using were safe and not like those nasty street drugs that ruin lives and cause addiction. That lie was deadly.
Without opiate prohibition, you can have honest educational programs that help people seeking opiates use them as responsibly as possible and you can provide available, nonjudgmental services for helping them reduce or stop their usage.
My understanding is that Purdue's claims about the time-release mechanism providing safety and anti-addiction benefits were true, except that an addict who's hoping to get high could easily defeat the mechanism by crushing the pill. So in a framework of drug legalization, I'm not sure how you could say they even did anything wrong; what's the issue with an easily defeated safety mechanism if you can just buy pure oxycodone directly from your local dispensary?
Without opiate prohibition, you can have honest educational programs that help people seeking opiates use them as responsibly as possible and you can provide available, nonjudgmental services for helping them reduce or stop their usage.