The supply chain funds warlords and criminal gangs. Impurities in the heroin cause abcesses and the need for amputation for injecting users. Restrictions on needles cause spread of blood-borne disease - and this also affect low wage workers who have handle sharps in public bathrooms and so on. People need money to feed their addiction and so they turn to prostitution[1] or acquisitive crime.
Treating heroin as a criminal problem has failed to protect people from themselves, but it also fails to protect wider society.
I do agree that we need to be cautious though. Look at the mess caused by alcohol. (That may be more true in the UK than the US.)