Legalizing marijuana would remove the need for users to get their supplies from criminals. The gangs that thrive on selling the drug illegally would find their customers disappear. Users would be getting hold of medically approved supplies that are less likely to have side-effects that might result in them committing crimes while under the influence.
Legalizing marijuana would not lead to an increase in crime. Instead, crime would likely fall, and the law enforcement could direct more resources to serious crimes. Prohibition showed what banning alcohol can do to people; turning many into criminals. Likewise, banning drugs has made many criminals that would otherwise be law-biding citizens. If the government wants to decrease crime, it needs to decriminalize marijuana.
I feel the legalization of marijuana would actually end up leading to less crime. The criminal acts that people have to resort to in order to get their hands on the substance with it being illegal leads to more crime. If people simply had access to it, I don't see any reason why crime would escalate.
Currently organized crime plays a large part in the cultivation and distribution of marijuana. By legalizing this plant, the government could take this source of revenue out of the hands of criminals. Additionally many innocent citizens have been jailed for possession of marijuana, some for long periods of time. The legalization of marijuana would eliminate these kinds of convictions and reduce the overall crime rate.