I have several psychometrics and psychopharmacology classes lodged in the nether regions of my brain, if that means anything to you.

Alcohol makes people (even those that aren't "violent") less inhibited. It interferes with their judgement, motor skills, and often makes people aggressive. All of these lead to situations where one is much more likely to commit or be the victim of a violent crime. You aren't going to find a psychologist say that anything Causes anything else generally, but you will hear them say that alcohol is the Best Explanation for punching a guy due to the increased aggression, poorer judgement, and lack of inhibition that occurs as a result of intoxication. It's also important to note that my statistics weren't citing alcoholics (people that have a "medical" history of alcohol abuse) but it was obtained by checking for alcohol in the system (physically or in oral histories) of the victim or criminal at the time of the assault.

As far as violent individuals go, it's a pretty ambiguous idea. The closest, of course, would be psychopaths, though their violence is more likely to be transactional and emotional than physical. Still, from what I remember and see with a quick review of the literature, they are not significantly more likely to abuse alcohol than the general population.

Finally, in regards to admitting the harmful consequences of alcohol but still arguing for the prohibition of marijuana, I have this to say: If we want our laws to be obeyed, they should be just. Something that is significantly more harmful to society should not be legal (alcohol) while something that is comparatively much less of a plague carry heavy penalties for use and waste tons of taxpayer money. Our laws should discourage in an appropriate manner that which is most harmful to society.