Don't confuse Vengeance with Justice. God forgives. The judicial system does not. He took nine innocent lives so he should have to pay the debt with his own.
You're confusing correction with punishment and legality with morality.
There is a reason that many other developed countries got rid of the death penalty a long time ago. It doesnt deter (death penalty states consistently have higher violent crime rates, actually); it doesn't rehabilitate (you can't rehabilitate a corpse); it's not cost effective (life imprisonment is less expensive for the taxpayer); offenders aren't always guilty (and if it means killing even one innocent, it should be banned); and it promotes the idea that violence and the taking of another life is okay as long as it is the government dishing it out (which in itself fuels the idea that the government, its offices and its officials and officers are somehow exempt from the same moral standards everyone elsemust abide by). It's utterly backwards and useless.