It seems to me that there have been in the past, and even the recent past, situations where the Government putting a person or persons to death seems appropriate, rather like a matter of self-defense; for example, gangsters in the 1930's who were terrorizing both everyday people and each other. Today we would call them terrorists. We then can easily add a person like Timothy McVeigh, guilty of mass murder. I think that anyone, a tyrant, a mob or an individual, who practices genocide without remorse, is a still a candidate for the death penalty - in some form. The death of Osama bin Laden did not break my heart. The self-induced death of Adolf Hitler was clearly appropriate, as was the execution of Saddam Hussein.
Bringing it down out of the higher government levels to individuals, McVeigh was clearly eligible, and I would put Ted Bundy in that category as well. Any serial murderer is a mass murderer from where I sit. These would be my exceptions to any "100% against death penalty" stance. *[edited: I have studied more since I wrote this.]
But here's the problem: once you start making exceptions like this, where do you stop? All of these were well documented and irrefutably proven cases, with multiple victims, witnesses and crimes. I would stop right there and extend the death penalty no further into the area of "individuals," because at that point you start getting too many errors. The most recent has been the Troy Davis execution in Georgia, who was most likely an innocent man.
This web site focuses on the Innocent Man and the wrongful conviction. (I believe Westerfield's case is both of these.) There have now been hundreds of death penalty cases in the U.S. overturned for all kinds of errors. If I am against the death penalty, it is more for these practical, ethical reasons than for moral/religious ones.
SHOULD human beings be allowed to kill each other or not? If you say "Yes, in these cases" and make exception for criminal executions and war, but you say "No, in these other cases" and call them "murder," where do such "exceptions" end? If I am against murder and against war, as a sort of practical pacifist, and I am against most criminal executions because they are so often botched, how can there really be any exceptions?