Drugs arent special as you say, but they also arent the cause of the violence. Gangs will still be violent and kill each other. It will just be over something new. So yes, we might see a decrease in drug-related violence (im still not convinced of this though), but we will see an increase in violence related to something else being traded on the black market.
I also have a hard time seeing crack or meth being legalized when many people still sue the cigarette companies for giving them cancer. I think tort reform is also needed.
You're ignoring that legal things, even when taxed are cheaper and safer than illegal things. Legalizing something kills the black market for that something. The reason is simple and obvious, real markets tolerate real business and real business out competes illegal business because it doesn't have to markup the product to make up for the overhead of being illegal which is what makes up the majority of prices for drugs on the black market.
Drug dealers don't tolerate competition under threat of violence and charge as much as they can and have to make up for product lost to law enforcement, real business has to compete with other businesses on price driving the price down to reasonable levels and also delivers a better safer product.
The illegal drug market will always exist, regardless of legalization. Why? Anything legalized will be regulated and taxed and many people will still find it easier to go around both.