I think the best argument regards mitigating circumstances that his lawyers could make would be that he is only a year or two older than the minimum age for the death penalty and he has been smoking weed regularly since before he was death penalty eligible until a short time ago.
If the argument for a minimum age for the death penalty is that the mind is not fully developed for full culpability, then the fact that
affected his developing mind and made him more likely to become overly religious and unduly influenced by his brother. The paranoia that comes with dope smoking can make a susceptible person think that he or his religion has all of the answers.
However, the aggravating factors of the numbers and scope of gruesome injuries and the callous disregard towards innocent people will outweigh that.
I think the only way he avoids a death sentence would be if he convincingly apologized and begged for mercy after explaining how he got to that point.
Dershowitz's statement about some crucial admission being excluded that would affect the outcome is off base. Even if some statements are excluded it will not matter.
You do not need a confession to prove that this was an intentional act with foreseeable and expected consequences and a confession is not needed to establish sufficient aggravating factors regards sentencing.
I am opposed to the death penalty for moral and church reasons. I also think that it is too expensive because the trial has to be done exactly right because the standard for death penalty defense lawyers is to preserve every issue and for death penalty appellate lawyers to argue every possible issue and then there are other stages of review beyond the first post-conviction appeal that take a long time and use a lot of resources if the prisoner wants his lawyers to try and save his life.