Since making something illegal doesn't make it go away, drugs are sold in a black market. Black markets have no legal protection, so gangs step in and protect their interests through violence.
It used to be the same way when alcohol was prohibited. The end of alcohol prohibition unsurprisingly resulted in a decrease in violence since there was no longer a reason for gangs to be involved (stuff like this is why we take history in school).
Ending the war on drugs will only decrease violence.
Also, marijuana doesn't cause people to become violent you dum.