"Gangsters are killing each other for the right to market cocaine, heroin, meth and more. Even if weed could be taken from the menu, they'd continue. They're bad people who enjoy being bad."
Of course legalizing marijuana would not magically transform hardened criminals into model citizens, but while "bad" is part of what it means to be a gangster it is not the reason why they became gangsters. There are whole host of reasons why people join gangs but by far and away the biggest draw is the money and the fact that the gangs are always hiring as it where. However, if there was a lot less product to move, the gang world will down size just like any other sector of the economy experiencing a down turn. The difference being that jail and death will do what a pink slip would normally do. Members who ended up in jail or who died or who simply moved on would not be replaced the way they once were. The less gang members there are the less shooting there will be.
Furthermore, as Canada is not a producer of coke and heroin, fledgling groups, robbed of the seed capital that marijuana profits provide, would have a difficult time breaking into the market. (Breaking into the marijuana industry is relatively easy and that is why there are so many small players. It is not just that it is grow locally ---and relatively easy to grow at that --- and so there are not the complications and costs associated with smuggling into the country. The demand is so much larger and it is spread over virtually every demographic in ways that demand for other drugs is not. People might not know a single person who uses meth or heroin, but they certainly know people who smoke dope, i.e., potential customers.) They would simply not have enough capital, cache and connections to survive. The less players there are the less violence there will likely be.
This brings me to another point. The experience of other countries has shown that the more profitable the gangs and the more international connections they acquire the more the gangs expand their operations as well as branching off in other directions. Marijuana is providing the seed capital the gangs need to expand and diversify. We need to nip this is in the bud.