Sure! They do anyway, or did you think heroin disappeared since it was banned?

It does need to be regulated. Doing it well will be difficult in a lot of places. I'd suggest modeling heroin sales after Nordic alcohol market: there's a single state-owned company that has a legislated monopoly, and no profit goals whatsoever. This makes it available, you can know that it's not mixed with anything cheap and deadly, and you also avoid anyone trying to push people to buy something that is quite obviously bad for them. I'm not saying it's an ideal situation that people do heroin, it is _quite_ destructive. But people do it anyway so let's make the best of that situation.

The harsh truth of reality is that people will make bad choices no matter what you do. Thinking you can ban something out of existence is naive and harmful. Best you can hope for is the entity selling drugs not using peer pressure to push it on kids who were only looking for weed. Which is what we have now if that wasn't clear.

To be perfectly clear, I do not encourage anyone to inject heroine, it's a terrible idea! Don't do it! But I'm realist enough to realise that some people are going to anyway, bans and common sense be damned,and I want to make that bad choice as safe as possible for them. I do not condone throwing drug addicts under the bus as we currently do, pretending that they're less than human and that they have no place in our society. I don't think the "outcast" status has ever helped anyone quit, in fact I'm quite sure it makes it harder. For example in Sweden being under the influence is illegal, so you could get in trouble for seeking help with your addiction. No wonder the druggies hide!

Well out of sight != Out of mind. At least not for me.

You should model yourself as a rational agent that makes mistakes sometimes. Suppose it is the case that, if you had heroin on your shelf at home, you would inject it on 0.1% of days. Maybe because you are especially sad/sick/in pain once every few years. If this were the case, you would not want to buy heroin and put it on your shelf at home. 99.9% of the time you will have the willpower to not inject it, but you still expect yourself to be addicted within a decade. The point of bans is to decrease that to 0.0001%, except for those who are actively seeking it.