I agree that the high cost of drugs (because they are illegal) leads criminals to engage in violence to protect their interests.
Making drugs legal may reduce drug related violence. I'm just not sure it would reduce over all criminal violence in the long run.
Repealing Prohibition ended killings over the liquor trade. It didn't end organized crime.
Would legalization make junkies less likely to rob or steal to support their habits? Or would a lower price simply mean that they spend the same amount of money but can now afford more drugs?