I already stated that one can be convicted on circumstantial evidence.
We are simply talking about sentencing guidelines and not about guilt or innocense. The legislature can determine any sentencing guidelines it wishes. It requires 2, 5, 7, 15, life, death, etc. based on the details of the crime.
They can pass a law that limits death sentencing to those guilty parties that have either 2 eyewitnesses and/or physical evidence.
Or they can have a new trial called 1st degree murder capital that requires 2 eyewitnesses and/or physical evidence. It can be the only trial eligible to impose the death sentence. If the prosecution doesn't have the requisite evidence, then they will prosecute under 1st degree murder life.
These are simply my opinions. They are designed to provide something near absolute certainty that innocent parties are not executed.
Just noticed your tagline. Thank you for you service.
By the time you get to the sentencing phase, guilt has been determined. If there are any reasonable doubts, there should be no sentence at all. This is not a civil trial where the ruling is based on a preponderance of evidence. A determination of guilty means you're sure he did it, not that you think he probably did. In the sentencing phase, you must decide if the crime committed is deserving of the death penalty, not if the evidence was sufficient to prove he committed the crime. There are many things to consider in the sentencing phase, but guilt or innocent is not one of them. If you've gotten to the sentencing phase without deciding whether the man is guilty you've failed miserably in your duty as a juror. If you've decided that he's guilty, but you've also decided the evidence wasn't strong enough to prove it, you've really screwed up.