On principle, I believe that a systematic method of the state killing a person is unethical.
I believe that it is better that 100 murderers walk free than to wrongfully execute an innocent person.
I understand that the practicality of executing a person as opposed to sentencing them to life without parole is long, cold, expensive, and difficult.
That said, I think the only cases that would merit execution are atrocities against humanity that reach a historic scale. The Nuremberg Trials, Osama Bin Laden, and not many other cases.