Sometime late in my career as a prosecutor, I came up with the Vowel Theory, a shorthand method of deciding which defendants were worthy of the death penalty. Under the Vowel Theory, you should not seek the death penalty against someone unless you have all the vowels.
A: Atrocious Crime. But aren't all murders atrocious? Yes, but in order to have a sustainable death penalty, the crime has to be off the Richter Scale of atrociousness. It has to be unimaginably wicked. Dousing someone with kerosene and setting match to him would qualify. Shooting someone once in the chest probably would not.
E: Egregious Fact Pattern: Not only must the crime itself be atrocious, the circumstances surrounding it must be horrific. If our kerosene dousing defendant belonged to a terrorist cell and did the crime in furtherance of his terrorist agenda, it would be egregious. If he was mentally ill and acting out on some insane delusion, it wouldn't.
I: Innocent Victim: Often victims engage in conduct which contributes to their demise. I've had several murder cases where the victim told the defendant "You don't have guts enough to shoot me." This tactic seems to work much better on television than it does in real life. Anyhow, such a victim is not likely to be an innocent victim.
O: Odious Defendant: The defendant has to be a really bad person, not someone driven to a one-time crime by emotion.
U: Undeniable Guilt: Simple proof of guilt beyond a reasonable doubt is not sufficient. You have to be able to prove guilt beyond beyond a reasonable doubt.
The cost in time, energy, effort, and money required to get someone executed is astronomical. The death penalty should be reserved for only the most unspeakably evil. I'm not saying that only the most unspeakably evil deserve the death penalty. I'm saying that society can't afford to execute everyone who deserves it.