Drug users are going to use drugs whether they're legal or not; legalizing drugs keeps users out of jail (helping to lessen prison overpopulation) and ensures they use as safely as possible (e.g. clean needles, pure/uncut substances, etc).
While illegal, drug users will hide their abuse and problems at all costs, making it harder for everyone to help them. Legalizing it will make it more accepted, making it easier to seek help for those who realize they have a drug problem.
Prohibition of a certain product does not eliminate its market, but only creates a black market. These markets can be regulated if we legalize the product and control it ourselves, and this would greatly deter organized crime and drop crime rates in general.
Some people are biological predisposed to be addicts. however, if you never try the drug you can't possibly get addicted to it. Having them out in the open will destroy millions of lives. Addiction is a mental disorder not bad morality
If production is legalized, then someone will begin producing it. Then we'll have businesses producing highly addictive drugs. This is an extremely risky situation for the country and world. The cartels may disappear but they'd be replaced by legal businesses. Less murder but more widespread use.