For me, it's about ensuring some kind of deterrence. Life in prison without the possibility for parole can be too soft when the crime is the intentional negligence leading to the deaths of thousands, if the punishment is life in a low security prison with access to TV/internet. If you have the connections to ensure you end up in a nice little apartment in Club Fed, you might not think as hard about the consequences as if the punishment was potentially death.
I'm also somewhat skeptical on the idea that the death penalty _should_ cost more than life in prison (assuming that sentence carries at least 20 years for the remainder of that person's life.) It would seem to me that there is a fairly obvious problem there that needs to get worked out if it's actually true (has anyone looked closely at the studies that assert this claim? Obviously there's additional time in court, but it would seem preposterous that this would necessarily lead to an increase such that paying for someone's housing and supervision for 20+ years is cheaper than additional court time.) Obviously there are other factors like how the government allows itself to get overcharged by the company that manufactures the lethal injection for some strange reason (is it actually all that strange to anyone?) And then what is the actual reason for all the time prisoners spend on death row, this would seem to just be the state shooting itself in the foot on statistics for the costs for capital punishment?
Anyway, I'm generally for the death penalty in matters where there is a clear connection between one entity and an intentional large-scale loss of human life. Proving the intention and the connection are the hard parts, for sure. But the clear benefit to the death penalty is that it's a permanent act that would-be criminals would naturally try to avoid, potentially avoiding them becoming criminals at all. If you have a study that somehow refutes this theory, that would-be criminals, especially white collar criminals similar to officials at 3M here, don't consider punishments as a deterrence at all, I might revise my opinion.