Okay, here we go.

If weed was legalized, the dealers would just move onto something heavier, like cocaine. Not only that, but the weed you get from the government would have to be regulated and it wouldn't be the same as the stuff you get on the streets, which would still result in dealers selling it illegally.

If weed was legalized, dealers would be shit out of lock for product to sell. They would not be able to compete with industrial corporations pumping out perfectly rolled marijuana cigarettes (joints), scaled operations, marketing/brand trust, and distribution. This wipes out tax dollars wasted on weed enforcement from police on the ground to clogs in the justice system, and in fact raises tax revenue for states (see California). If dealers decide to switch to serving harder drugs, they would only saturate existing cocaine/meth/heroin markets. Thats a big IF, because those drug productions are not as simple as growing plants and are significantly harder to enter at a big scale.

We also don't have any really good long-term studies regarding pot. Cigarettes cause cancer. Do we really want to legalize another substance that does the same thing or worse?

We have thousands of years of marijuana usage in humans. We have studies, just uneducated people.

Marijuana fights lung cancer:

"The active ingredient in marijuana cuts tumor growth in common lung cancer in half and significantly reduces the ability of the cancer to spread, say researchers at Harvard University who tested the chemical in both lab and mouse studies."
http://www.sciencedaily.com/releases/2007/04/070417193338.ht...

Marijuana fights brain cancer:

"THC, causes brain cancer cells to undergo a process called autophagy. Autophagy is the breakdown of a cell that occurs when the cell essentially self-digests.
The team discovered that cannabinoids such as THC had anticancer effects in mice with human brain cancer cells and people with brain tumors. When mice with the human brain cancer cells received the THC, the tumor growth shrank."
http://www.webmd.com/cancer/brain-cancer/news/20090401/marij...

Marijuana fights breast cancer:

"A compound found in cannabis may stop breast cancer from spreading throughout the body, according to a new study by scientists at California Pacific Medical Center Research Institute. The researchers are hopeful that the compound called CBD, which is found in cannabis sativa, could be a non-toxic alternative to chemotherapy."
http://www.foxnews.com/story/0,2933,312132,00.html

Marijuana unlikely to cause cancer.
http://www.webmd.com/smoking-cessation/news/20000508/marijua...

You also seem to shift the blame of the violence on the government. So, it's the government's fault that kids decided to sell pot illegally, make shit-tons of money, and attempt to get another dealer killed (and get killed in the process). They could have somehow magically prevented the situation by legalizing pot.

An economics 101 class would do wonders for anyone and everyone. When something is illegal to have, there is a risk of being caught. When theres a risk of being caught, you need incentive to put your ass on the line. When the incentives are big money and fast money, people start getting crazy. Gangs and organized crime get involved, and shit goes down. Why aren't gangs killing each other over tobacco and alcohol dealing?