One thing I'd like to add, which some guides omit.
Currently, Sodium Nitrate is currently under investigation for use as a peaceful mean of feral hog control in parts of the US. Considering how sancrosanct animal comfort it in the US, it would appear there is something about SN that can give a peaceful death.
As always, do your best to find as much anecdotes and research. Don't rely upon those who simply say, "I tried it, and it the worst pain I ever had. It caused permanent scarring, gave me an extra nipple, and caused me to develop a prehensile tail." Look at lots of stuff from many resources, compare and contrast and make an educated best guess.