DNA isn't the only evidence that can confirm that someone is guily. There are finger prints and CCTV cameras being found with the murder weapon and eye witness testimonys. If we just relied on DNA then it would be much harder to get convicted. You probably won't get DNA in a shooting so its like saying that if you kill someone with a gun you won't be executed.
Perhaps because we watch so much crime television, people often want very clear-cut DNA answers about who committed a crime. But this isn't always practical, because there's not always that type of proof left behind at a crime scene. In fact, it's only in recent history that this science has advanced so much. Circumstantial evidence is another kind of evidence, and sometimes there can be an overwhelming amount of it, such as someone purchasing suspicious items at a store and being seen on camera with them. We shouldn't require DNA evidence for conviction of a crime.
Yes, individuals found guilty of murder should be allowed to be sentenced to death if there is not DNA evidence that they committed the act, because DNA is not a magical catch-all for crime. If there is enough evidence to convict the person, there is enough evidence to punish them accordingly.
I don't care if the person was caught in the act, I still would not support the death penalty. I certainly hope we aren't sending people to their death without DNA evidence. To me that proof would be paramount even in giving a life sentence, much less a sentence like the death penalty.
If there is no concrete evidence of a murder, I do not think a person should be sentenced to death. A life sentence, perhaps, but definitely not death. In science, eyewitness evidence is the worst form of evidence. In law, it's the most often used. This has to be changed.