I think the death penalty is a necessary part of the concept of justice. If the victim's life is taken, why should the criminal's life be spared? Is the criminal's life more valuable than the innocent victim taken away from his/her family? Not to mention that where you have a truly dangerous individual, shouldn't the other prisoners be protected from him?
While the criminal justice system is not perfect, it is designed in such a way that it is much easier for a guilty man to go free than for an innocent man to be imprisoned.
But to be honest, even DNA evidence is not as accurate as the 1 in billion numbers we hear everyday. The labs where the testing is done subjects the results to human errors... ie. tainted samples, other conditions interfering with the results. To read the studies of the labratories that have to be shut down because of the conditions shows how misleading even DNA evidence can be. Anyway, sorry for the tangent.