Ah, so heinous crimes would be given the death penalty.
Because sentences are handed down by either a panel, jury or judge a justice system can never be flawless. Human error, emotions, experience, as well as cultural and ethnic biases are all factors that could give the wrong sentence to the wrong fellow. Even with a strong appellate court system, bad cases slip through the cracks. A simple case study of Texas executions over the last fifteen years will reflect that.(Just trust me)
That said no Justice system will ever be flawless no matter how complete or good it is. Therefore there is no chance that an alleged criminal will always be treated the same. And as every murder case is different there can be no precedents to set standards. The sentence of death has been and will always be an imperfect science and people who probably shouldn't get the death penalty from time to time be sentenced to just that. So since there can never be a flawless justice system you cannot always be 100% sure when sentencing a person to death.
Another note: Why not accommodate the innocent? After all they are who the Justice system in any country is there to protect.