Capital punishment is hotly contested for a reason: it is very emotional. One pro argument is that it will give justice to the families of the victims of the worst crimes. That argument supports the idea that some people are too evil and dangerous to be allowed to live. One con argument is that the death penalty is irreversible. If a mistake is made and an innocent person dies, there is no way around it because we cannot bring people back from the dead.
Capital punishment is one of those hotly debated issues in which few peoples minds are changed. It is such a deep seated emotional issue that it is almost impossible to maintain civil discourse.
The typical argument to support Capital punishment is the heinous criminal, such as Charles Manson who deserves nothing less than death. My problem with it is, how does one separate this from revenge, unless, of course, one considers revenge a proper element of society. If Capital Punishment is justifiable, why is it normally done secretly, often at night, and with a certain solemnity and reticence?
Perhaps the strongest argument against capital punishment is almost all western societies have abandoned it as a crime against humanity and uncivilized. The United States is the only Western country to still practice it. Ironically, the U.S. was also the last Western nation to abolish slavery. Perhaps there is a lesson to be learned there.
One reason to support capital punishment is that it is important for people in a society to feel that the government is protecting them. If they feel that people can commit terrible crimes and not have a proportional punishment, they will lose faith in their government. If you look at it this way, capital punishment might be vital for some countries as a way to keep the people trusting their government.
Locke said that the people consent to be governed so that their lives, liberty and property would be protected. If the government does not seem to protect their lives strongly enough (by killing those who take life), people might be less willing to consent to be governed and society would be weakened.