Are you actually talking about something more akin to date rape? I'm really not sure if I understand how that might work. You slip someone opioids without them knowing until they're addicted and then you stop but reveal to them they're addicted to opioids hoping they continue that addiction? Or do you stop once they're addicted and just hope they figure it out? Or you just keep feeding their addiction without them knowing until you feed them a dose they OD on?
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.
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.
i.e. it was something done to you without informing you. To use your analogy it would be like blind folding someone and pushing them off a cliff (with base jumping gear on) if they lived I'd consider that at least attempted murder. But with secretly giving drugs to someone there is no way to avoid interference with the workings of a sound mind so that I think is on par with murder.