Looking at TV drama's like the wire to shape an opinion on the drug war is in no way even remotely close to thinking the premise of dexter is desirable in real life.
If the War on Drugs is a miserable failure, couldn't you say the same about a criminal justice system that allows a lot of really bad people to prosper. Even if they are captured,convicted and sentenced to death(in states that allow this), the prospect of the sentence being carried out is a pretty distant reality.
Consider the states that don't allow the death penalty. Serial killers get life in prison at taxpayer expense.