I am against the death penalty in most cases. If you are talking about cost, it costs more to put someone to death than to imprison them for life due to the expense of the appeals process. I feel it is justified when the evidence of guilt is overwhelming and the crime is extremely cruel and their are no mitigating factors.
In this case the evidence was overwhelming, and the crime was extremely cruel, but I feel there were a few mitigating factors. He was fairly young and current scientific studies show that the brain of a young adult of 18 is not fully developed, particularly when it come to judgement and impulse control.
I also think he was greatly influenced by his brother and would not have done this on his own. He was also influenced by his religion and everyone here knows how religious beliefs can warp your thinking. I also think putting him to death would make him a matyr and possibly influence others to follow his example.
I would have chosen life in prison without the possibility of parole. That's a lot of years for him to think about what he has done and what he lost by taking this course.