Legalization of marijuana will clearly decrease crime.
First and foremost, criminalization of marijuana use in specific and drug use in general is a failed strategy. It's attempting to treat what is fundamentally a mental and public health issue as a crime control issue. We've had decades of research, including numerous RAND studies, that indicate repeatedly that rehabilitation programs and similar more-compassionate approaches work far better than "source country control" or imprisonment.
It makes sense. People aren't choosing drug use for rational reasons. They're satisfying needs, whether it be self-medicating for conditions like an anxiety disorder or just dealing with some desire or need within them. A deterrent based on fear isn't going to make that fundamental need go away. The best solution is to satisfy the need directly.
Even if you just have a clinic that gives out suboxone or methadone or even straight up gives out high-quality heroin with a safe place for someone to inject, that clinic is going to bring people there who every time they come are going to have a chance to see a Narcotics Anonymous group or get literature for help.
Unsurprisingly, then, all sorts of studies have repeatedly indicated that decriminalizing marijuana is either benign or actively beneficial for crime reduction. Medical marijuana has been net positive in reducing crime. International analyses confirm domestic analyses as well. While it may be a bit unfair to compare the U.S. to the Netherlands, it is still unquestionable that the countries in Europe that have the lowest rates of recidivism and the most enlightened crime policy all treat drug control as a social issue rather than a criminal one.
It's astonishing when people try to argue that legalizing marijuana will worsen crime even as they have to admit that doing so starves the cartels. When legitimate growers and distributors emerge, and they have the police to protect their property from theft or damage or sabotage, the cartels have no chance. Why would you ever buy from a sketchy drug dealer with an Uzi when you can go to a nice store in town that's playing some Doobie Bros. on the PA?
And remember: Nothing prevents regulation of a legalized product. One can still deter against excessive drug use, say by having sobriety tests when one is pulled over or allowing employers to drug test.
Legalizing marijuana helps addicts get help instead of needing to turn to crime and stay within a criminal subculture to get their fix, breaks the power of dangerous drug cartels, and promotes an entire legitimate sector of the economy that will let a benign mind-altering consumer product comparable to alcohol or caffeine be sold by legitimate vendors paying their taxes and complying with the law. Private business will promote employment, leading to lower rates of unemployment, while public institutions will actually benefit from a massive economy that they at present can't tax. Police officers will be able to focus on actual crimes instead of trying to stop people from using one of the most popular mind-altering chemicals in human history. The notion that all of these effects would somehow be eclipsed is absurd. Marijuana is even more benign than alcohol: it doesn't tend to promote violence or criminal behavior. If decriminalizing alcohol unquestionably reduced crime, how would decriminalizing marijuana fail to do the same?