All that would still be very illegal (I imagine injecting someone with even a completely safe saline solution without their consent would be considered assault in most jurisdictions). The type of people that are comfortable doing that clearly don't care about the law. I just don't see drugs being illegal as a limiting factor on crimes of that nature in today's society.
I'd also like to mention that a major source of funding for many criminal gangs is the black market sale of drugs. If we legalized and regulated drugs, they'd lose that source of money and I'd personally expect to see a decline in organized crime.
It’s the action of adding chemicals to your brain that can and likely do have harmful long term effects, not limited to addiction, and many still unknown. If such an act was done to me I would consider it within my rights to kill that person in retribution.
I’m less confident on the expected drop in organized crime as taxes on the drugs usually means there is now money in tax evasion. Without tax it may become so cheap that other problems start to dominate.
As for illegal drug sales persisting despite legalization, how many people are still buying illegal moonshine? It does exist, I've seen it, but it's not nearly the problem it was during prohibition.
I’m more worried about organized crime/political corruption than I am about freely available drugs, but organized crime has all sorts of ways to make money so I don’t think we’ll be rid of it that easily. I think people underestimate the prevalence of how much corruption still exists. I even think it’ll get worse as fewer non-compromised people want to take police jobs.
Also prohibition ended 1933 but the major crackdown on gangsters was in 1970 due to a long period of federal government inaction.
Criminal gangs tend to take the more active holding people down and injecting people with drugs approach than the slow sneaky process. But once addicted the addict is now reliant on the gang for their continued supply and can be controlled. The real life instances I’m aware of was done on people who were preparing to testify against their family members in a gang. It was a speedball injection. But is has been depicted in fiction such as in the move ‘the town’.
My relatives were involved in sheltering them to get them clean, but as soon as they went home the gang found them and got them addicted again.