It was also said despite all that there was a causal relationship.
From the same link
What makes this new study more compelling than previous studies is that the researchers followed the same individuals for over 50 years from a young age to adulthood. This is precisely what one needs to solve the chicken or egg riddle with respect to cannabis and violence: just look and see which one happens first.
Hers's what they found: Most of the participants never used cannabis and they were never reported to have violent behavior. 38% of the participants did try cannabis at least once in their life. Most of them experimented with cannabis in their teens, but then stopped using it. However, 20% of the boys who started using pot by age 18 continued to use it through
middle age (32-48 years). One fifth of those who were pot smokers (22%) reported violent behavior that began after beginning to use cannabis, whereas only 0.3% reported violence before using weed. Continued use of cannabis over the life-time of the study was the strongest predictor of violent convictions, even when the other factors that contribute to violent behavior were considered in the statistical analysis.