> of some bad actor sending out code which turns those cars into ungoverned highway missiles

Yes but think of the probabilities that have to happen are:

a) Knowledgeable person

b) Motivated person

c) Evil or mentally unstable person

d) Determination and ability to overcome adversity.

And the person or person who has to decide that Tesla is what they want to exploit instead of another car manufacturer or another object and so on.

Compare this with someone who simply wants to take their car and drive it into a crowd.