Should capital punishment be legal?

Capital punishment, also known as the death penalty, is a government-sanctioned practice whereby a person is killed by the state as a punishment for a crime. The sentence that someone be punished in such a manner is referred to as a death sentence, whereas the act of carrying out the sentence is known as an execution. Crimes that are punishable by death are known as capital crimes, capital offences or capital felonies.

Currently, the legal status of capital punishment varies by region. Should it be legal? Under what circumstances?

Pro

 * Life in prison is arguably no more humane, but harder and more expensive to implement.
 * The "no more human" is relative to what you consider "more human". Life in prison, if it is "humanized", could help to achieve the objective of prisions, which is to punish somebody while re-educate and preserve human dignity.
 * For the family and friends of a murder victim, the suffering of losing a loved one may never end. However, for some, the execution of the murderer provides a kind of closure that allows them to move on. For them, justice will never be served until the murderer is put to death.
 * There are a lot of people who would want the death penalty for a lot of other crimes, even (relatively) trivial crimes. Justice is not about just following the victims' wishes.
 * This is a false assumption. It isn't the death penalty, but more likely the efficiency of the legal process, that provides closure to the victims loved ones.
 * The justice system attempts to punish proportionally to the crime. If the death penalty is not an option, then murder will probably be met with life prison. But there are other crimes that are met with life prison, like severely hurting someone. So if the punishment can't get any worse, what would stop criminals from delivering that final blow and killing its victim? Death penalty is the ultimate warning. If citizens know that premeditated murder can lead them to death, they are much less likely to commit it.
 * There can be worse crimes than killing someone, for example killing more than one person or torturing before killing. If the punishment can't get any worse, what would stop criminals from killing other victims after the first one, or torturing before killing?
 * This argument might open the debate on whether a worse punishment than death sentence should be applied in some cases. For example, for those who tortured before killing somebody, a proportional punishment could be to apply some kind of suffering before the execution.
 * The death penalty has been with us for almost as long as murder. If its goal is to dissuade people from premeditated murder, then it isn't working. Some countries with capital punishment have a high murder rate, while other countries without it have a low murder rate. If anything helps to prevent murders, it isn't the fear of capital punishment.
 * The causal relationship is not clear just by what you've written. Perhaps those nations believe capital punishment is necessary because they have a high murder rate.
 * Keeping murderers in prison for life is very expensive. It isn't fair that citizens are forced to pay for their maintenance if they aren't providing any useful service.
 * Prisoners can provide useful services from prison and thus pay for themselves.
 * Yes, but evidence should be provided on whether prisoners are able to pay for the imprisonment working at the prision. Besides, what if prisoners refuse to work? Shall they be executed then?

Con

 * If punishment is meant to rehabilitate criminals, then the death penalty can't be an option, simply because dead people can't learn!
 * The main goal of punishment is not to rehabilitate criminals; other goals include punishment (which some will reject as a goal), and deterrent (which some will deny to work). It is not the main goal especially for the penalty for one of the most severe crimes one can commit--premeditated murder.
 * The death penalty doesn't work as a deterrent, and the goal ought not to be to simply indulge the feelings of a victim's loved ones.
 * Some will see the indulgment of the feelings of a victim's loved ones as a valid goal.
 * In fact, one main argument exposed here is that death penalty does work as a deterrent.
 * The right to life is a fundamental human right (Article 3 of The Universal Declaration of Human Rights). Defending this right by executing murderers is hypocritical and even contradictory.
 * This is a moral debate, not a legal one. This argument is akin to saying that premeditated murder should be punished by death in Texas because their constitution says so. The Universal Declaration of Human Rights, like every other legal document, is based on moral arguments. If arguing shows that premeditated murder should be punished by death, then the declaration may need amendment. The fact that some people at some point in history reached the conclusion that the right to life is inalienable, doesn't imply that they couldn't have been mistaken, just like so many other authors of legal documents have been throughout history.
 * Life in prison is sufficient punishment for any crime. Death is unnecessary and excessive. Death may even be a relief for many murderers who lived a terrible life outside prison and an even worse one inside the prison.
 * This argument is contradictory. If death is a relief for some murderers, should they pay their crime suffering in a horrible prision then? In that case, death would not be excessive, but the most human punishment.
 * There seems to be a disagreement as to what "punishment" and "relief" entail to each person in the process; one man's punishment is another's relief, if you will. We may exact execution to a murderer expecting it to be punishment because, if we were in their position, we'd see it as such - but this doesn't necessarily entail that they equally perceive death as punishment. In fact, if the murderer who willingly accepts execution as relief from a miserable life projects their perceptions onto their victims (just like the executioner who expects their deed to be seen as punishment rather than relief), who's to say the murderer didn't believe they were doing a mercy unto their victim supposedly living a miserable life like themself? If we remind ourselves that - like the masochist who "errors" in finding pleasure in what is considered not pleasurable - we all hold subjective perceptions, then the notion of a person providing treatment to another which, if delivered to the self from someone else would incite a positive reaction, and thus motivate said-person to enact what to them is a service, becomes plausible. The (ironic) common denominator in all this is that we selfishly expect others to see the world as we do, though our acts are disguised as services unto others, service here warranting transparency for whether it is kind or evil is, well, dependent upon subjective perception and experience.
 * Capital punishment is expensive and thus a huge financial burden to the taxpayers due to the extensive legal process required. In the US it currently costs more to execute someone that to put it in jail for life.
 * Guillotines are probably the most humane type of execution, as one can be killed very quickly and painlessly. They also likely only require minimal maintenance, and if so, it should be a very cheap method of execution.
 * Capital punishment is irreversible. Both judge and jury are people, and it's inevitable for people to make mistakes. If someone is wrongly convicted and sentenced to life in prison and later found innocent, he or she could still be released and properly compensated.
 * It is impossible to compensate people for time lost and other hardships they experienced from the legal process. People do make mistakes, but this should not undermine pragmatic means of addressing intractable anti-social behavior like premeditated homicide. Someone guilty of premeditated murder (especially more than one instance) may be a danger to other prisoners being kept for less serious offenses. Putting them in long-term solitary confinement is inhumane and unlikely to improve their disposition.