I've been exposed to every type of pothead. I know the majority of people who use weed don't experience psychosis, but there are those that do and when smoking weed they're a danger to themselves and society, so I believe in keeping it illegal.
I've been around for awhile, and I, too, have known all types of pot users (I'm from Brooklyn). But I have never known, or known of, anyone who became psychotic and dangerous to others or themselves from using marijuana.
Methamphetamine, yes. Alcohol, most definitely. Marijuana -- absolutely not.
The only effect I've ever seen or been aware of from using marijuana is the diametric opposite of violent behavior. In fact, there is no more effective tranquilizer than THC -- which is one reason why the pharmaceutical industry contributes to NIDA. If marijuana is legalized one of their most profitable (and addictive) products will soon be pushed aside.
I suggest those whom you believe were made psychotic by pot were in fact reacting to some other drug, or some existing psychosis, you were not aware of.