The difference is that cocaine use is only really harmful to the user whereas murder obviously involves a third party.
You also have to consider the impact of the policies themselves. Cocaine prohibition may reduce cocaine addiction but it also creates legal issues for cocaine users that are often more harmful than the drug itself in addition to creating the space for a criminal blackmarket to exist and thrive. On the other hand, murder prohibition reduces the number of murders and is generally considered to be worthwhile.
You also have to consider the impact of the policies themselves. Cocaine prohibition may reduce cocaine addiction but it also creates legal issues for cocaine users that are often more harmful than the drug itself in addition to creating the space for a criminal blackmarket to exist and thrive. On the other hand, murder prohibition reduces the number of murders and is generally considered to be worthwhile.