> Isn't murder survival of the fittest?
Murder is bad for society, as people who can benefit society through say their knowledge of medicine or physics aren't adept at self defence.
A similar argument could be made that people who aren't very good at taking care of themselves are valuable to society somehow and should be protected from idiotic self-harm but I believe a person's autonomy trumps society's interest in preserving whatever value they have.
> I don't want cough medicine that might stop my breathing, but heroin was regularly sold as just that before the FDA.
Heroin was sold as cough medicine that might stop your breathing? If it tells you you might die and you choose to ingest it, that's on you. If you choose to play Russian Roulette, nobody's stopping you.
In case it wasn't sold with a warning that it might stop your breathing, you could have chosen to verify that it did what you expected before ingesting it. The modern FDA could also warn of the dangers without forbidding the sale of the substance.
> Tobacco products are heavily regulated.
They may be regulated but any adult can still choose to partake of however much they like regardless of the damage to their health.