Decrease the size of the industry that can't access state violence via the courts to solve it's petty business disputes (i.e. the illegal drug industry).

A huge amount of violent crime of all types in the US is a figment of drug distributors and dealers, and to some extent users robbing, shooting and beating each other to settle scores.

Hell, just making possession completely legal (none of this "civil infraction" bullshit) so that people can go to the cops when they're robbed instead of getting revenge on who they think done it would be a huge step forward.