Not being able to decisively turn off the engine has also killed people, remember the Toyota issue not long ago? We think that in general the drivers panicked and floored the accelerator thinking they were pressing the brakes, but the complexity/difference of turning off the engine with a button---as I recall it had to be held down for something more than a second---vs. turning a key all the way counterclockwise and ultimately pulling it out could have resolved the user errors short of fatal crashes.
Ditto the theory that some or many of these were caused by the accelerator getting stuck on a floor mat. Or an embedded computer and/or components attached to it failing in a bad way.
Compare to the big red buttons on some computers and hopefully in all big machine rooms: there are times, when lives are threatened, that you want an easy way to immediately turn everything off. Even if you add a molly-guard in front: https://en.wikipedia.org/wiki/Big_red_button#Molly-guard