marijuana, like a number of other drugs, can be disinhibiting, and in that way might lower one's threshold towards acting violently. It's not likely that it will make him an all-out and thoroughly violent person, but he needs, while not under the influence of it, to look seriously at how it affects him, as viewed by observers like yourself qwho may not be under the influence themselves at the time. He should re-evaluate whether his enjoyment of it is worth this side-effect ( all drugs have side-effects, whether pills or leaves, swallowed or inhaled ).