perhaps one could alternatively say the violence is black market related
I really wish that we had good stats on this stuff so that the laws could be crafted to minimize violence. Anyone who says we're civilized and "beyond that" isn't paying attention, it's just that the violence has largely been outsourced so that a portion of affluent society can feel that we're "beyond that".
Gang members seem to have no regard for enforcement of any kind and throw their lives away with abandon, but I still think less enforcement will only embolden them.
There's a school of thought that we inadvertently create terrorists by interfering in the middle east and thus motivating people to want to do us harm. And by 'we' I mean 'The United States'.
I suspect that a similar argument can be made that the illegal drug trade and gangs are fairly well correlated and that gangs arise because individuals don't derive any security from the police for a variety of reasons. So gangs are the result of police policy, not that police policy is the result of gangs.
I'm not suggesting that this is a 100% solid theory. But there's probably some nugget of truth in it.
Then there is racketeering/blackmail, which is probably more of a problem of ineffective policing, at least as far as the "Hey, you've got a nice business here, how about you give me a cut to make sure nobody comes by [sic buy] and smashes your cabbages" variety goes.
By your reasoning, though, one might surmise that ending the drug prohibition would then also end gangs, wouldn't one? The gang exists to defend illegal drug territory. Illegal drug territory only exists because drugs are illegal. Ending prohibition would make drugs legal, not illegal, and thus the outsized profits would go away. This would then make operating the gang unprofitable and it would disband or "go out of business".
I'm sure you'll bring something else up like "but drugs are bad!" or some such. But that line of reasoning works, doesn't it? Changing the subject because you don't like the moral implications of legal drugs isn't an argument.
Forgive me for accusing you pre-emptively of moving the goalposts. It happens a lot and I get tired of it. That's usually what happens once it becomes clear that legalization would end gang violence, another objection to stopping legalization for another, never before mentioned reason.