Except it being "properly administered" is not the issue, the issue is, is it ever proper for it to be done?
If the Zyklon B is the right dose and it is poured into the gas chamber through the correct gas entry port, that is irrelevant to the propriety of the act.
The Arkansas law deals with the propriety of the act, not the methodology - and it is EXACTLY a Tenth Amendment reserved power of Arkansas to legislate about.