We deal with fatality based odds every second of the day. It’s all background noise until it’s your timeColt (or maybe S&W) used to have a print ad that warned you to keep an empty chamber under the hammer for just this reason. In particular, a lot of old revolvers (and semi-autos) aren't drop-safe. That's why safeties like firing pin blocks and transfer bars exist today.
Odds are that it was the result of a mistake, but you can't eliminate mechanical failure based on the facts we have.