Is it though? The framework to give out that kind of judicial analysis is based on historical evidence, and death sentences are far from being historically unusual or of particular cruelty (not all of them at least).
She lured, tortured and killed a teen; I really don't see an argument where the sentence she received is unfair, or cruel. I think there is a strong argument against the death penalty when we know justice sometimes condemn innocents, but she's not of those cases, she bragged of the murder and carried around a piece of her victim's skull as a boast.