Answer-
Retribution is something given or demanded in repayment, especially punishment and demanding someone’s death is itself a kind of violence and unexpected in a civilized society. Death penalty is a kind of retribution or revenge based on acts performed by someone. It has been observed that sometimes government use Death penalty as tool to take the revenge. This type of practice is more common in developing and under developed countries. In developed countries such as USA and UK, never use such type of practices.
Killing someone in order to take revenge or vengeance is not ethical. If someone is given death penalty, he has to die due to the desire of the victim or his loved ones to take revenge. It can be compared to the act of revenge performed by the accused.
There will be no difference between the accused and the judicial system if their acts are based on retribution and these acts of them lead to the death of someone even if the person who is punished is a criminal himself.
Civilized society should look forward to implement moral values in all humans including criminals. If someone performs a crime, not only that human is responsible but also the society is indirectly responsible which could not teach its own member, the learning of love, respect, forgiveness. There is no guarantee that by killing the criminal, crime will also diminish.
Also, death penalty is not compatible with human rights and human dignity. Most basic of the human rights is the right to live. Death penalty violates this basic right of humans. Death penalty also violates the right of humans that they can’t be tortured and subjected to other cruelties. It also leads to inhumane treatment or punishment of humans by other humans. Death penalty undermines the human dignity which is inherent to every human being. Life is sacred and killing is always wrong whether by individual or by the state.
There are always chances of innocent people getting executed. Despite of every effort made by judicial system and use of sophisticated and high tech methods, it will always remain susceptible to human failure. No one can be sure that no innocent person will get punished by mistake. If there is no guarantee, then death penalty should not be there to avoid death of many innocent people.

There have been incidences in the past and present also where death penalty is used in disproportionate manner. It may be used against poor, minority, and members of racial, ethnic, political, and religious groups.
It has been indicated by the General Assembly of the United Nations that there is no conclusive evidence that death penalty is effective in decreasing number of crimes. An increasing number of law professionals are continuously questioning the effectiveness of death penalty in order to prevent crime.
These is a lot of public support for the death penalty but can’t be considered as a block for abolition of the death penalty. There are strong evidences that there had been gross support for the acts of human right violation in past which was later on condemned vigorously. It is the understanding of the human rights and human values that guides us what is right and what is wrong. Compatibility of the death penalty with human rights and human dignity should be accessed.
References: http://www.icomdp.org/arguments-against-the-death-penalty/ http://www.religioustolerance.org/executb.htm
http://www.bbc.co.uk/ethics/capitalpunishment/against_1.shtml
http://martinfrost.ws/htmlfiles/scottish_anatomy/capital_punishment.html
http://deathpenalty.procon.org/view.answers.php?questionID=001004