The usual standard is to find a person guilty or not guilty based on the evidence, and if the verdict is guilty hand down an appropriate sentence based on the crime committed. It is not standard practice (or at least it didn't used to be) to find a man guilty when you're not reasonably sure, and then hand down a lighter sentence than you would if you thought the man was guilty.
I don't agree with the current law.
I believe you should be allowed to convict on circumstantial evidence within a reasonable doubt.
However, the death penalty must not be enacted without at least 2 eyewitnesses and/or hard physical evidence. I will settle for life imprisonment in those cases.