According to one former police officer, the effect of drugs themselves are not the main cause of violence:
…drug users aren’t responsible for violence–high people just want to enjoy their high. Drug violence is business violence….the largest causes of drug murders are territorial disputes over market share. If no one will help you defend your property–you can’t trademark the winning name that you’ve given your drugs; you can’t take a competing drug dealer to court when he tries to take over your corner; you can’t sue for libel if someone says your product is defective…
This comes from the book Cop in the Hood, by Peter Moskos. It is interesting throughout.