Yes, people are regularly convicted on circumstantial evidence and people are (not as regularly) exonerated after years in prison. Imprisoning innocent people because the prosecution got it wrong or even worse falsifying or withholding evidence is terrible but fixable. Once a death penalty is carried out there is no fixing that. IMO executing an innocent is too big of a risk to take with our justice system.
No, I think Roof committed those murders but still don't believe he should be put to death.