Most murders are committed during times of great fear or emotional stress or under the influence of drugs or alcohol. When the crime is premeditated, the individual rarely believes he or she will be caught or executed. The death penalty is a lottery. With 20,000 homicides per year in the U.S., about 4,000 are convicted of murder and of these 250 are sentenced to death. Sentencing depends on dozens of random factors including the skill of court-appointed defense counsel and the prejudices of the judges and juries.
Some judges hand down more death sentences than others.
Some states, for example Florida, have drastically more people on death row than other states, adjusting for differing population densities and murder rates. The death penalty is no way to show that killing is wrong. Justice does not mean that punishment must imitate the crime. Imposing the death penalty does not bring back a murdered loved one. It is no compensation for the loss.