There are two main reasons why there is a high level of violence in the illegal drug trade. First, it is very lucrative. Second, it is illegal.
When the trade in a certain good or service is lucrative, there is a great deal of competition to fill the demand for that product. We see fierce competition in every field of industry. It is no different in the drug trade. However, since drugs are illegal, the competition turns violent. There are no lawsuits over patents. There is no competition through branding and marketing. The major form of competition is violent.
If the drug trade were less lucrative and/or more legal and regulated, there would be less violence associated with it.