Not peaceful at all.
You can find accounts written by people in the 90s and early 2000s who failed ctb via cyanide when it was a common method. It appears to be incredibly painful if one is conscious enough to remain cognizant of what is happening.
I remember reading about the death of David Conibear. He had to buy some sort of equipment to test his cyanide solution, because if the PH is even slightly too acidic it will literally burn a hole through the stomach.
To me, that sounds incredibly horrifying. There is the risk of lasting damage if one survives, not to mention the strict penalties for possessing the substance in the first place.
This is one of those things, like the urge to eat 300 paracetamol, that you can't reccomend against enough. Very risky and not peaceful in the slightest.