Hydrogen cyanide was one of my alternative methods I had considered, but have decided against it. I have no firsthand account of what the pain is like, but I remember reading somewhere (sorry, I can't remember the source; it's been a while) where a person had ingested potassium cyanide capsules and tried to record what the pain was like before losing consciousness. Took a few minutes to kick in, but when it did, he described it as his whole body being lit on fire, from the inside. It's not instant asphyxiation unless the concentration is really high, but even so, it could be a whole minute or two or three feeling that kind of pain before dying. Unless one of your other alternative methods is self-immolation, I don't know if this would be a good route to go. Pain appears to be just as agonizing.