Violence is definitely not triggered by marijuana use, despite the many myths originating in early hemp prohibition propaganda circa 1937. In 1982, the United States National Academy of Science reported in “Marijuana and Health,”
Both retrospective and experimental studies in human beings have failed to yield evidence that marijuana use leads to increased aggression. Most of the studies suggest quite the contrary effect. Marijuana appears to have a sedative effect, and it may reduce somewhat the intensity of angry feelings and the probability of aggressive behavior. 
Many large sociological studies indicate that violent and sexually aggressive behaviors are directly related to consumption of alcohol.  Television is also implicated in violent behavior in impressionable youths and elderly people.  Violent individuals admitted to hospitals are much more likely to have alcohol, cocaine, caffeine, amphetamines, PCP, or other stimulant drugs in their system than marijuana. 
Related sections: Psychoactivity, Stress Reduction.
 Gieringer, “An overview of the human research studies on medical use of marijuana.” 1994, Source: CANORML, http://www.norml.org/canorml/
 World Health Organization Project on Health Implications of Cannabis Use: A Comparative Appraisal of the Health and Psychological Consequences of Alcohol, Cannabis, Nicotine, and Opiate Use, II. “The Probable Health Effects of Cannabis Use.” 1997
 Johnston, “Elderly mixing up television, reality: Study: Violent images on the tube disorienting seniors.” London Observer, Source: San Francisco Examiner, May 26, 1998
 Conversation with double board-certified psychiatrist Francis Podrebarac, MD, 1998-1999