An eye for an eye creates a world full of blind people.
I was a huge supporter of the death penalty. In the past couple of years I have changed my mind.
As a species we have used state sanctioned execution since the beginning of recorded history. It is not a deterrent. Taking the offender's life doesn't bring the victim back to life nor does it end the suffering of the loved ones of the victim. Therefor execution doesn't balance the scales of justice. That is the problem with human justice; it is imperfect. There are many wrongs that cannot be made right.
In addition, I do believe in redemption. That is not to say everyone who does wrong will redeem themselves, but I do believe they should be afforded the opportunity.
Finally, I feel uneasy with sentencing someone to the death penalty for crimes where there is no eye witness other than the deceased and the accused. Also, the "Felony Murder Rule" in the United States presents an interesting moral dilemma. 3 men conspire to rob a bank - 2 go into bank and 1 is the getaway driver. 1 of the 2 robbers inside shoots and kills a bank teller. All 3 men are sentenced to death.