Why should anyone care who is to blame?
The question we need an answer to is "How do we fix the problem?". Culpability is completely irrelevant.
The quickest and cheapest way to reduce violent crime in the USA would be to decriminalize drugs. So that's the smart thing to do.
There are always ways to use violence for personal gain, no matter what system is in place. So it matters greatly that we emphasize and enforce blame for any efforts to use violence for personal gain.
There are also always ways to alter the system to make such opportunistic violence less likely, yet many of these changes should not be made because they require violating other important principles. This is another reason to not simply focus on what features of the system enable such violence, and always emphasize that the persons engaged in such violence are THE problem and must be responded to and viewed as such, regardless of what structures enable them to profit off such chosen violence.
In this case, it happens to be that the drug laws themselves have no valid justification because their illegality prevents little that is bad and prevents something good (individuals being able to choose for themselves how to treat their own body and mind). If their were legit positives of restricting these substances (as their are with guns, pollutants, toxins, etc..) , then the incidental impact of criminalization on use of violence would not be a reason to decriminalize. Thus, the real valid argument for decriminalization is not that criminalization enables more violence but that there is no valid positives served by the criminalization that outweigh the negative of infringing personal liberty.
So, yes we should decriminalize drugs, but the fact that some murderous scumbags use criminalization as an opportunity to use violence for profit is not the reason to do it. We need to go after such scumbags anyway, since they will seek other opportunities to profit that infringe on other people's rights to life and liberty.