This study shows that legal marijuana "led to a sharp reduction in violent crime in US states that border Mexico. While the Mexican cartels smuggle other drugs such as cocaine, heroin and metamphetamine across the border, the market for marijuana is the largest drug market in the US and the one from which the cartels can make the fattest profit. It costs around $75 to produce a pound of marijuana in Mexico, which can then be sold on for $6,000 depending on the quality."

Unless you're buying that 8th from a grower directly, it is likely linked to violence somewhere along the line.