There is no proof that the death penalty deters further violence. Virtually none. The only reason it exists is for a biblical sense of revenge.
I agree. When I took Law classes, I did my research paper on the Death Penalty, and I can confirm with the data I collected, that there is no correlation between the number of crimes committed nationally and the number of executions nationally. I had a graph I made of it, but it's gone now.
On a separate note, states that allow the Death Penalty in the United States tend to have MORE violent crimes committed every year. Crime statistics can easily be found with the slightest bit of research. So if there is even the slightest correlation between crimes and executions, then by these numbers, the Death Penalty theoretically INCREASES violent crimes.
So based on my research, there is no logical reason to allow the Death Penalty, but on a human level, there's even more reason to disallow it. First thing is the 8th Amendment, which protects US citizens from cruel and unusual punishment. What is more cruel than ending someone's life?
Also, the Death Penalty tends to be supported more by Conservatives, who favor smaller government. Yet they want to give the government the ultimate power over life and death? That is blatant hypocrisy.
On a fiscal level, the Death Penalty is also a bad idea, as it costs more to execute someone "humanely" (an inherently bullshit pretense) than to keep them in jail for life.
The Death Penalty also denies people due process of law. There have been many cases in history where people were found guilty of crimes which later they were proven innocent. The Death Penalty is the only irreversible punishment, and denies people their basic Constitutional rights.
Justice, not vengeance.