No, there definitely is. There are plenty of cases where guilt is a 100% certainty. People have videotaped their crimes, admitted to it, and have their DNA all over the crime scenes. There is no room for any argument against their guilt. For instance, there is no question that a guy like Russel Williams(Canadian serial rapist/murderer) is guilty. There is zero doubt. Where it gets dicey, is a guilty verdict being considered final and conclusive in every case, including cases built entirely from circumstantial evidence, where there is room for reasonable doubt, even after the verdict has come down. For instance, I don't believe the Death Penalty should've been on the table for a guy like Scott Peterson, who is likely guilty, but still has room to make a case for his innocence.
I think there is a way to hand out death penalties to the 100% guilty people, while avoiding potential mistakes with others who are found guilty, but aren't in the same area of definitive guilt.
I can see the moral argument against the Death Penalty, and if that's why someone is against it, then fine. I think a middle ground can be found with the other argument though, and there are ways to avoid making mistakes while still handing it out to people who are without a shadow of a doubt 100% guilty. You don't need to get rid of it outright. They just need to adjust the standards to meet such a punishment, and they should be extremely high.