In the two years since Colorado legalized the recreational use of marijuana the overall crime rate has dropped by more than one percent and property crimes dropped 3.1 percent. The over the same time frame homicides dropped by 12.8 percent. In fact the ONLY area there has been an increase in crime in Colorado is in arrests for public display or public consumption of marijuana in city parks.
In particular, drug related murders, gun trafficking, thefts, and kindnapping go down when marijuana is legalized. Legal marijuana means legal methods of growing and distributing it. That means less turf wars, less of a need for "protection" and less worry about other people stealing your product or ratting you out to the authorities.
The legalization of marijuana helps educate people about drugs. There is a big difference between using marijuana and other narcotics. Highly addictive drugs like heroin and cocaine are much more harmful and likely to lead to criminal behavior. Making a clear distinction between drugs that are relatively safe to use like marijuana and drugs that are dangerous helps people who want to experience drugs make better choices. With more people choosing marijuana over more addictive substances there should be a decrease in crime caused by addicts' need for the next fix.
Legalizing marijuana leads to a decrease in other crimes. First, governments no longer lock up non-violent drug offenders. Sending many of these poor people to prison actually turns them into hardened criminals. Secondly, many crimes are committed by people trying to skirt the drug laws in our country. Finally, legalizing marijuana will free up law enforcement resources that can be used to fight more dangerous crimes.