Application of the death penalty must meet two criteria, in my book:
1. Capital crimes with a body count higher than one.
--Murder one person, prison for life should be the maximum penalty, in my view. Murder two or more, that's a mass shooting or terrorist or war-crime type situation where death should be on the table.
2. There should be zero - zero - doubt of the accused's guilt.
--Criminal convictions only have to satisfy a "beyond reasonable doubt" standard. It's hard to quantify, but in my mind, that means an 80% or more probability of guilt. However, for the state to apply the death penalty, the probability of guilt should be 100%. Not 99.9%. 100%. In other words, reserved for Dylan Roof-type mass shootings where there are some or all of the following: multiple witnesses, video footage, caught-red-handed by the cops, a confession, and zero remorse.
Reserved for extreme scenarios like this, applying the death penalty helps bring closure to victims and to the rest of society, who can be assured the perpetrator won't somehow get out of prison on a technicality or a jailbreak, or simply live out the rest of their days in prison as a kind of human totem, potentially inspiring other sickos to commit the same or similar heinous acts.