In effect, the criminalization of drugs may be a cause that results in the violence associated with the drug trade. This is not unlike the criminalization of sex trades, and appears to parallel the US experience with the Prohibition of alcohol. It would be rather overoptimistic to assume that the problems would go away if drugs were decriminalized. I would nevertheless suggest that these problem areas reflect an unfortunate codependancy between governments and criminal activities. Drugs are expensive because they are illegal.
Criminal activity is encouraged by the resultant high prices.
Violence is encouraged by the competition for sales at those high prices, combined with the fact that since the transactions are not legal, participants cannot entrust their safety to government authorities. If you put a drug dealer in jail, you have created a job opening. Discourse on Political Economy and the Social Contract - Rousseau John McCarthy of Stanford University holds some interesting opinions on the issues of Ideology and Sustainability.