I agree with you, I would prefer to be dead than be housed in a cell for 23 hours a day.
I think the cost for execution is MUCH more than a life in prison. But most of it is due to the prolonged legal battles, reviews, requests, more reviews, hearings, etc. A bunch of lawyers at $500 an hour over twenty plus years adds up!
It would seem to me that with these mass shootings the perpetrator is usually pretty clear who it is (conspiracies not with-standing). So I would like to think a short trial could be expected. (Aren’t most of them killed during the response anyway?)
Baloney sandwiches and mac ‘n cheese will slowly kill them off anyway. Nothing but a restrictive life until you die.