We the people have more important things to do with our hard-earned tax dollars than feed, clothe, house, guard, and provide flat-panel plasma TVs for the most vicious killers, child rapists, and the like ad infinitum.
It costs more to keep someone in prison for a year than it would to send a deserving young person to virtually any college in the nation for the same length of time. However, executing a criminal needn't cost more than a few dollars for rope, which is easily reusable.
The issue is with the justice system and not the death penalty.