> It beats me why they don't just sedate these people and simply chop off their head or so.
Because there is more risk of suffering. Sedation step might not work right. It also disfigured the body for a proper funeral and is messy. A single gas that has both sedative and lethal effects is the lowest risk and most humane of any options. Particularly when its biologically simple so has no potential for drug interactions or an immune system response.