I'm of two minds on this. When you have a guy like John Mohammed, the DC sniper, or Dzokhar Whats-his-name, one of the Boston Marathon bombers, there's no question that the death penalty is the right way to go.
But I also think that there are many cases where an over-zealous prosecutor has sought the death penalty in cases where it wasn't so clear.
All in all, I think that it's better to err on the side of caution. There have been too many cases where death-row inmates have been found to be innocent after spending years and years waiting their turn to ride ol' sparky.
And there have certainly been cases where the guy was found to be innocent after he had the ride to eternity... even though the powers that be have done their best to cover it up.
Oh, and I almost forgot... What about the case in Oklahoma where the lab technician fudged the test results on a whole bunch of cases just because she thought it was "the right thing to do", and dozens of guys were sent to prison as a result of her manufactured and/or doctored evidence?