Just shoot them in the freaking head. How painful would that be? They'd be dead immediately. And that would save the state a lot of money too. How expensive is one bullet and a janitor to clean up the mess?
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." -Manuel II Paleologus
Too messy.
Better would be to run a morphine drip. 1 mg per minute for about 30 minutes ought to do it.