Death is the ultimate punishment. There are no do-overs. The death penalty probably constitutes the most intrusive form of government action currently permitted by law. On the other side of the equation, life is without question the most fundamental right afforded to Americans by our Constitution. It should not be taken from us without the highest level of justification. Accordingly, the death penalty should not be imposed as a sentence unless (1) the crime is truly heinous, and (2) there is absolute certainty regarding the perpetrator's guilt.
Such a rule would generally limit the penalty to imposition on those with a long history (established by non-circumstantial evidence) of engaging in inhumane action -- serial killers, mass murderers, genocidal tyrants and the like. I am completely fine with that. Attaching death as a potential penalty for "ordinary" murder raises far too great of a risk of inconsistency, discrimination, and plain error.