If you want to play with chemicals, carbon monoxide is way too merciful. Sure, if you place them in a glass chamber you can watch them slowly suffocate, but if you're making a point of making him suffer, try sarin, or organophosphate, or maybe even a large amount of mustard gas for immediate effect. The best part is that sarin and organophosphates are legal chemical pesticides, so no one would suspect you're out for torture/murder when you're innocently purchasing a few gallons of the stuff.
My room is an insane asylum, and I am the patient.
Beware of Dog signs are overrated. Beware of Writer.
Warning! Crappy author at work! Any hapless bystanders/passerbys will be sentenced to an eternity of hell by eye-hurt :3
Some people are hourglass shaped, some people are apple shaped, some people are pear shaped, some people are even bean pole shaped, but if we’re not careful we all become banana shaped. This is your gentle posture reminder. — Ranger