Public opinion polls show that nearly two-thirds of Americans support the death penalty, but that is a significant drop from the peak, in 1994, when 80 percent of respondents told Gallup pollsters they were in favor of capital punishment. When asked if they would endorse executions if the alternative sentence of life without parole were available, support fell to 50 percent. [Emphasis mine].
Forgetting about the philosophical argument of right or wrong in theory, the practical matter is that our system is not capable of providing 100% certainty that its outcomes are correct. And because death is different from other forms of punishment, we cannot continue to practice the death penalty and risk the execution of innocent people. Of the 1000 people executed since 1976, how many have been innocent? I'd guess at least 1% - 10 people. And perhaps as much as 5% - 50 people. Is the killing of the other 900+, as opposed to keeping them locked away for life without parole, really worth the stain on our conscience that killing innocent people brings?