They seem like good reasons to me, aside from the fact that killing is wrong.

From: Death Penalty

Capital punishment does not deter crime.
Scientific studies have consistently failed to demonstrate that executions deter people from committing crime. The respected Thorsten Sellin studies of the United States in 1962, 1967 and 1980 concluded that the death penalty was not a deterrent.



The USA is unable to prevent accidental execution of innocent people.
The wrongful execution of an innocent person is an injustice that can never be rectified. Since the reinstatement of the death penalty, 119 men and women have been released from Death Row....some only minutes away from execution.



Race plays a role in determining who lives and who dies.
Race is an important factor in determining who is sentenced to die. In 1990 a report from the General Accounting Office concluded that "in 82 percent of the studies [reviewed], race of the victim was found to influence the likelihood of being charged with capital murder or receiving the death penalty, i.e. those who murdered whites were more likely to be sentenced to death than those who murdered blacks."



The death penalty is applied at random.
Politics, quality of legal counsel and the jurisdiction where a crime is committed are more often the determining factors in a death penalty case than the facts of the crime itself. The death penalty is a lethal lottery: of the 22,000 homicides committed every year aproximately 150 people are sentenced to death.



Capital punishment goes against almost every religion.
Although isolated passages of the Bible have been quoted in support of the death penalty, almost all religious groups in the United States regard executions as immoral.



The USA is keeping company with notorious human rights abusers.
The vast majority of countries in Western Europe, North America and South America — more than 117 nations worldwide — have abandoned capital punishment in law or in practice. The United States remains in the same company as Iraq, Iran and China as one of the major advocates and users of capital punishment.



Executions are carried out at staggering cost to taxpayers.
It costs more to execute a person than to keep him or her in prison for life. A 1993 California study argues that each death penalty case costs at least $1.25 million more than a regular murder case and a sentence of life without possibility of parole.



Millions could be diverted to helping the families of murder victims.
Families of murder victims undergo severe trauma and loss which no one should minimize. However, executions do not help these people heal their wounds nor do they end their pain; the extended process prior to executions prolongs the agony of the family. Families of murder victims would benefit far more if the funds now being used for the costly process of executions were diverted to the provision of counseling and other assistance.



Incompetent Counsel is a Persistent Problem
A study at Columbia University found that in 68% of all capital cases reviewed between 1973 and 1995 the state and federal courts found errors sufficiently serious to require retrial or resentencing.



There is a better alternative.
California judges have the option of sentencing convicted capital murderers to life in prison without the possibility of parole. There are currently over 2,700 people in California who have received this alternative sentence which includes a limited appeals process. According to the Governor's Office, only three people sentenced to life without parole has been released since the state provided for this option in 1977, and this occurred because they were able to prove their innocence.