I don't care about the death penalty as a deterrent. I care about it as a way to deal with certain people who will continue to pose a threat to others and for whom there are no other viable or humane options to rehabilitate them or otherwise render them safe to be around other people.

You can deal with a greedy pharma exec by seizing assets, injunctions against them participating in the pharmaceutical industry or serving in particular roles, stricter laws, regulations, and oversight on the pharma industry as a whole, etc.

You can't just regulate someone out of being dangerously emotionally disturbed, and if they can't be rehabilitated, what other viable options do you have left to keep them from harming others? You can lock them up in solitary, or perhaps take some drastic medical interventions like sedating them, lobotomizing them, or I guess chopping off their arms and legs, and at the point you're considering those sorts of options, the ethics of just killing them start to sound much more attractive.