Is there any doubt that the injections used to put sick dogs and cats to "sleep" are essentially painless? If so, a lot of pet owners would want to know it. If not, would that be a simpler approach than the multi-stage "lethal injection" for condemned humans, now the subject of several court cases?
Related question: why do states insist that lethal injections be administered by the one group -- medical professionals -- whose code of ethics prohibits doing harm? I suppose it'd be impractical to have "executioner" as a non-medical technical career -- there wouldn't be enough work to keep anyone fully employed. One hopes.