But people abuse alcohol and look what happens there.
MJ is rarely linked with violence, the only reason it is seen as bad is because of it's illegal status. People think that because it is illegal it must be seriously messed up stuff.
If anything MJ actually makes you feel really chilled out and therefore LESS likely to commit acts of violence. The majority of MJ users only smoke it every now and then and therefore don't have much of an incentive to go out and commit acts of violence in order to obtain it.
Also in proportion to the number of users more people probably drive under the influence of alcohol, and driving under the influence of alcohol is more dangerous. I know a couple of people who have used weed in the past and they found that it actually helped enhance their concentration.
People shouldn't drive under the influence of ANYTHING fullstop, so I'm not condoning it, but alcohol related deaths are far higher in proportion to the number of users. Also I'd like to point out the number of people a year who are involved in traffic accidents due to side effects of LEGAL medicines (such as painkillers or antihistamines).
Tobacco and alcohol especially are far more damaging both to health and society.
Also if they legalised it, that would be taking it OUT of the hands of dealers, therefore people wouldn't feel the need to resort to petty crime or violence in order to obtain it, they could just go and buy it in an off-licence.
Generally most of the population don't associate MJ with gun crime. When the word "marijuana" is mentioned the majority of people think of a group of chilled out hippies, whereas if I say "alcohol" loads of people immediately think of disorderly or violent behaviour, such is the effect of this drug.
MJ is also not a gateway drug. The majority of people who use MJ for its effects are happy sticking to it, and have no desire to progress to more harmful illegal drugs. The ones who do take other illegal drugs alongside, probably would have taken these drugs anyway regardless as to whether or not they take MJ.