There are many reasons for this. Among them are:
- Opposition to the death penalty on moral grounds. Some people feel that it is simply wrong for society to kill people, regardless of what they have done.
- Lack of effectiveness. Some people argue that the death penalty is not a good idea because it does not effectively deter people from committing murders. They feel that the point of the death penalty is to reduce crime. If it does not do this, it is not worth it to have capital punishment.
- Inconsistency in application. Some people think that the death penalty would be okay if we could figure out a fair way to determine who gets sentenced to death and who doesn't. They point, for example, to studies that say that people who kill white people are executed more often than those who kill black people. This sort of thing makes them feel that people are getting sentenced to death relatively arbitrarily, based on the attitude of juries, not on the objective facts of what they have done.
For me, the narrative of Cain and Abel in chapter 4 of the Jewish and Christian Bible is the best argument against capital punishment. In this narrative, Cain slays his brother Abel. He does so with premeditation -- it is Murder One - as we hear in TV court dramas. There are no mitigation circumstances surrounding this account of murder in the dawn of humanity. Yet God -- the God of the Old Testament -- does not slay Cain or have him slayed. Instead he lets him live. God does not let Cain live out his life in a biblical "Club Fed" instead Cain's life becomes a sentence of hard labor, scorn, and much pain but also of redemption. Cain becomes a builder of cities and his offspring become leading inventors and craftsmen. The blood of Abel which out for justice is indeed granted that justice but justice without vengeance.
Cain has time to redeem himself even after committing such a great crime as taking the life of his own brother. If capital punishment had been applied, that teaching from the Bible suggests a world eve more violent than today's one lacking the gift of music. Yes, one of Cain's sons, Jubal, developed string instruments and pipes.
Some people are against the death penalty because they have been desensitised to the actions that would have triggered the death penalty in the past; therefore considering the death penalty to be a violent action not warranted in a civilised society.
Some people are against the death penalty because they focus on the cases that suggest that defendants have been wrongly punished with the death penalty - such as the Derek Bentley case in 1953 ( see the BBC news on the day of the hanging and then the over-turning of the conviction in 1998) They therefore give the greatest consideration to the idea that someone who is innocent may be punished by death. Whereas it is possible to overturn a sentence and the prisoner be freed this cannot be done if the death penalty has been used.
Some people are against the death penalty because of the society in which they live. British society has stripped people of the steps that would have made the death penalty a natural consequence - such as corporal punishment in schools and the right for parents to physically chastise their children in any way.
Some people are alienated by the idea that killing someone who has committed a heinous crime makes you as bad as them. There is a need for some people to distance themselves not only fromt eh perpetrator but also the crime so to resist the reaction of using the death penalty is to resist the crime and the perpetrator,
Some people truly believe that justice involves penalties and consequences but the death penalty is more about revenge than justice.