Death penalty: is it okay to kill?

Even though punishment by death has been abolished in many places globally, there are still fifty-five countries that practice the death penalty, with the USA and China being among them. So what are the arguments for and against the death penalty?

• “An eye for an eye” is an old law that goes back to the times of the Babylonian Empire. It was written somewhere around 1800 BCE. However, death penalty supporters argue that people who have committed the worst crimes (e.g., murder) should be punished by death. On the other hand, the opponents say that justice systems are not perfect. The state would kill innocent people in the case of mistakes (which has happened in history). On top of that, the death penalty is the most popular in authoritarian states (such as China and Saudi Arabia), where it is difficult to argue against authorities' decisions. 
• The death penalty prevents crimes. The supporters believe that potential criminals will think twice before committing a crime if they know they could be executed. However, recent studies doubt this, and 88% of criminologists do not believe that the death penalty stops crimes. 
• Costs. The proponents claim that it costs too much for taxpayers to keep prisoners alive. However, the research shows that executing people is far more expensive. For example, California reports that the annual cost of the death penalty is  $137 million per year in the state. 
• Morality. The proponents claim that killing is justified if it follows a proper process and is done in order to punish crimes. On the other hand, the opponents say that killing is always morally wrong, and asking the police or medical staff to execute people as part of their jobs is unfair and damaging to their mental health.