Note that the public in California favors keeping it on the books — rather interesting for a pretty left-leaning state that hasn't executed anyone for years.
From a utilitarian perspective, I think it's a waste of resources. We know how to deal with absolute incorrigibles without killing them: put them in a dungeon and minimize human contact. This is what we did with Tom Silverstein and he never killed another staff member or prisoner after that. Expensive, but probably cheaper than trying to execute him. And while he put on a good show for the cameras and appeared stoic, I'd have to think his lot in life served as a deterrent to at least some others with nothing to lose who contemplate violence against staff or other prisoners.
Of course, then you've got monsters like Jaime Osuna. There's no deterring that beast, just managing it.