It's ineffective at reducing crime rate. It is the heaviest (scariest) sentence, but the US keeps its crime rate as one of the highest in the developed world.
America has one of the highest crime rates of the developed world.
People on death row have been convicted due to science that was deemed good at the time of the conviction, but then later science evolved and the science, was found to be faulty and now considered exonerating. In fact sometimes bad science in general can result in false convictions.
It's wrong to respond to injustice with injustice. Killing is wrong (a necessary condition to justify the death penalty), but it is also wrong to kill in response to killing. As a society we should above those who hurt us, rather than lower ourselves to their level of morality.
There are felons that cannot be rehabilitated (serial killers, people with mental disorders that have committed murder) and the death penalty should be considered for these groups as they suck up resources that would be better allocated elsewhere.
By whose judgement? Certain cultures have the death penalty as a punishment when breaking the law. To them, the death penalty is a right thing to do.
There is irrefutable evidence of direct deterrence in the fact that dead people cannot kill again. What's to stop a life w/ out parole prisoner from killing someone else without harsher punishment?
First Assistant U.S. Attorney Robert Troyer (“When lifers kill in prison, is it a waste to prosecute them?”, The Denver Post).