The death penalty is costly do to the appeals process. A murder trial will normally takes much longer when the death penalty is an issue. Litigation cost includes the time of the judges, prosecutors and public defenders, as well as court reports and the high cost of briefs, all which are paid by the taxpayers.
The death penalty alone imposes an irrevocable sentence. Once an inmate is executed, nothing can be done to make amends if a mistake has been made. There is considerable evidence that many mistakes have been made in sentencing people to death. Many of the releases of innocent defendants from death row came about as a result of factors outside of the justice system.
DNA testing has death row inmates from blame. Here, too, the justice system had concluded that these defendants were guilty and deserving of the death penalty. DNA testing became available only in the early 1990's, due to advancements in science. If this testing had not been discovered until ten years later, many of these inmates would have been executed. And if DNA testing had been applied to earlier cases where inmates were executed in the 1970s and 80s, the odds are high that it would have proven that some of them were innocent as well.
Society takes many risks in which innocent lives can be lost. We build bridges, knowing that statistically some workers will be killed during construction; we take great precautions to reduce the number of unintended fatalities. But wrongful executions are a preventable risk. By substituting a sentence of life without parole, we meet society's needs of punishment and protection without running the risk of an erroneous and irrevocable punishment.
With weakening due process it will also increase the deterrence and create fear of committing a crime. Some people probably, abstain from murder because they fear that if they committed murder they would be executed. Hundreds of thousands abstain from committing murder because they look at it with horror.