Respectfully, I am not so sure ... here in Colorado, most of the marijuana sold in dispensaries is between 17-24% THC by mass (as confirmed by mass spec) and many smoke it in large quantities. The highly potent THC extracts (known colloquially as "glass" or "shatter" or "dabs") are also for sale legally here as well and likewise are sold in large quantities on a daily basis. We also have marijuana edibles that provided standardized and regulated doses of THC and cannabinoids which are clearly marked; 10 milligrams is a standard dose and the amount recommended to tourists, but I've seen/heard of people eating >200mg in a sitting.
With that said, I've never seen marijuana in any of these forms cause anyone to become violent, ever, either through my preceptorship or just through living life in the Mile High City. We do get a decent amount of ER patients who come in because of weed.... usually because they are tourists or otherwise cannabis-naive and eat/smoke WAY too much. Usually one of 3 things happens:
1) They freak out and have a panic attack and win themselves some xanax and a healthy ER bill.
2) They have a syncopal episode (pass out) and/or vomit and sometimes hurt themselves in the process (fall and hit their head, etc)
3) They get behind the wheel of a car and do something stupid and come in with new best friends from the Denver Police Dept.
More or less in descending order of frequency.
However, one thing I have heard of - but never really seen or heard of in Colorado at least - are synthetic cannabis analogs (so-called "Spice" or "K2", among a million other names). These substances are usually some harmless plant matter that's been sprayed down with a liquid suspension of one of hundreds of possible laboratory synthesized cannabis analogs that have names like JWH-073, WIN 55,212-2, and CP 55,940, among others. These analogs are usually manufactured overseas in labs, put into a liquid suspension sprayed onto potpurri or whatever garbage smokable stuff manufacturers have on hand, and are sold in head shops under a lot of different names. They exist in a weird quasi-legal state and are very poorly regulated here in the US.
These substances have a much higher "affinity" for the cannabis receptors in the human body than regular THC. To use an analogy, if regular THC sticks to the cannabinoid receptor with the strength of a fridge magnet, these synthetic analogs would have the strength of an electromagnet used to lift and move cars. Because of this high affinity, they have lots of adverse effects, including inducing states of psychosis, causing seizures, strokes, vomiting, panic attacks, heart attacks, hypertensive crises, etc etc... they are in sum really terrible for you, and for sale at your local head shop in all 50 US states. Because they aren't really regulated because they aren't really illegal, the quantity and identity of the synthetic cannabinoid analogs in each bag of "Spice" or whatever varies from brand to brand and even from packet to packet within the same brand.
Here are a couple good articles couple articles from the UK perspective on synthetic cannabis analogs and a good rundown in Slate on the same topic.
Hope that was informative... sorry if it was too rambly or off topic.