Alcohol is not a CAUSE of violence. You cannot prove that people who are violent with alcohol will not be violent without alcohol - A controlled experiment on that scale would require controlling for every other factor which causes violence, which is basically impossible. And highly illegal.

I will admit there is a distinct correlation between the two, but it proves nothing. Perhaps an inclination to violence causes alcoholism. Perhaps just people who aren't drunk get away with more violent crimes, whereas people who are drunk get caught more, leading to a disparity in the statistics Linolaques mentions. You cannot prove that alcohol causes violence.

I also believe that Marijuana/hemp/whatever should be kept illegal. From an economic point of view, making it illegal increases the cost of getting the drugs and the risk associated with it, making it far more difficult to obtain. Should the drug be legalized, the price would go down as the drug cartels start sending in larger amounts of the drugs legally, and demand would go up, resulting in many, many more people using drugs. This might be okay to you, but I do not think it would be beneficial to the society as a whole. Therefore, I say no.

It is not a matter of whether or not Marijuana or whatever is worse or better than any legal/illegal drugs, it is a matter of whether or not the effect of legalizing it would have enough benefit to outweigh the harm.