As illegality of drugs labeled "bad" by governments continues, a black market thrives as people can't walk into a store and legally buy the particular substance. It's completely logical that if a substance is legalized, crime related to the illegal drug market will decrease basically to zero. A lot of violence caused by gangs are due to this market.
I believe that if certain drugs were legalized that the presence of crime and gangs would drastically decrease and actually cause some of our penal systems to decrease the amount of inmates they hold which in turn would save taxpayers dollars and have a positive effect on the economy if they are able to be taxed.
Yes, legalization of drugs would reduce the crime rate in a variety of ways. For starters, by taking a bunch of laws making something illegal off the books, a lot less people become criminals overnight by eliminating simple possession infractions. If a controlled, well-regulated, system of dispensing substances were developed the local street gangs big source of income dries up. It also helps fight crime and terrorism abroad by cutting off warlords and drug kingpins major source of funds. The people that will suffer will be those caught in a cycle of addiction. But I think we've seen under prohibition that people with addictive personalities will find a way to get what they want no matter how stiff a penalty you put on it . With the right safeguards in place legalization would more people's lives better globally than worse via addiction they are already prone to.
If you expect drug addict to reduce his crimes if you decriminalize drugs or legalize it then your pretty much misguided. Why because he wouldn't nessery have the money he would need to buy it so as we all know for drug addict its always about the next dosage, person in not frame of mind will do anything to get the next dosage. Black market wont go way ether there huge black market for lot of stuff that are sold legally such as clothes, sporting tickets, video games, movies etc . So anyone thinking legalizing drugs will solve the black market is seriously misguided.