In the majority decision for the SCOTUS case Hatuey posted, two of the major reasons for disallowing the death penalty for minors included a general understanding that peer pressure is harder for a teen to avoid and a limited ability to avoid situations that lead to criminal behavior may make minors more likely to commit crimes they would otherwise not have committed. I think both of those facts are completely irrelevant in the case at hand (again, IMO), as this girl acted on her own, planned the crime, and committed it on her own (with the exception of tricking her sister into coaxing the victim over).
At 15, the socio-emotional skill set is well developed and she would have already reached the developmental point at which empathy springs up. She would have been perfectly capable, barring socio/psychopathy, of understanding the implications of her actions and was under no pressure to commit them.
Now, that isn't to say we should willy-nilly apply the death sentence to those who commit murder...but I think an arbitrary exemption on the basis of age should be weighed very heavily against the facts of the crime before we start weighing sentencing options.