Legal marijuana will have the opposite effect.  There will still be homicide and crime because the hard and addictive drugs will continue to drive that.  People who partake of legal marijuana won't be a problem in general.  The individuals who are involved in heroine use and other hard drugs will most likely use marijuana as well.  They will also use anything else they can get their hands on.  People who like to smoke an occasional joint are easy targets for folks like this, but they aren't the problem.