Death most often has pain associated with it.
Maybe, but barbary in inflicting death is not a virtue. That is what this very ciminal inflicted, on his helpless victim, and without conscience. His ways are not a template for administering Western law, of course.
These chemical injections have been on the total fail list on many occasions for a long time, with complaints galore from those having to administer the darn things. Methinks they need to return to swifter, more certain methods for the death penalty. Just my opinion.