The wrong people making the wrong decision
It doesn't really matter what the techies at Mercedes, Ford, GM, Telsa or any other car maker decides. The rights and wrongs of the matter will be decided country-by-country in case law, as each injury-accident or death is prosecuted.
This will, or course, be a shambolic maze of conflicting principles, examples, precedents and exceptions. Not only will each car, from each manufacturer, have to have its autonomous software updated to account for they ways each country's laws evolve with each new case won or lost, but that same software will have to be aware when the car moves from one jurisdiction into another, and then start to drive according to that place's laws (just like a person would have to).
I can also foresee mass disabling of vehicles (by the million) when updates fail. You might think that it's annoying when W10 decides, of its own volition, to make your computer unusable for hours while it downloads some updates. Imagine when you try to get into your car to go to work and the dashboard informs you that it won't be drivable for another 2 hours due to a legal upgrade. (Or worse: when it pulls in to the side of the road to do the same thing, while you're traveling).