Let me jump in for a bit - I don't support the death penalty in its current form, and even one innocent life is too much of a cost, I think. The way that I do the moral math in my head is that getting rid of a person with no value to society is essentially an economic decision of sorts. So losing one innocent as the cost for a million guilty people is an unacceptable trade made to basically lift an economic burden.
On the other hand, I'm not entirely with the "no murder, ever" crowd, because in the case of people like mass murderers, they don't fit any of the criteria for my reasoning as to why killing is bad. They don't have any remorse, they won't become better people, they have no value for the lives of others, etc. In this case, we're not trading a worthwhile, redeemable life for economic reasons - we're just making a pure economic decision. Removing that person from society is therefore quick and has no negative repercussions. It's the monsters we don't want, and we don't want to foot the bill until they die a natural death. So we expedite it, and nothing of value is lost.
Of course, this doesn't work for the death penalty for a variety of reasons, including the lack of omniscience and the system being a form of state-sanctioned revenge. It's not meant to simply remove the cancers from society for its betterment - rather, it's used to represent the rage of the people. That's one of its most significant problems, I think.
Some people, however, simply deserve to die on the basis of being evil and hampering society. I don't lament the loss of life for those who don't care about it. But again, I'm not omniscient, so who can really know?
TL;DR: Some people deserve to die, but we can't know who those people are for sure. So while I don't think no one should ever be killed, I don't think that we have the capacity to know who the people that deserve to die are in most cases that are given the death penalty. Furthermore, the death penalty is more of a system built on revenge rather than utility. It is for these reasons that I don't support the death penalty. That, and even one innocent life is too high a cost.