Current software is unreliable and crashes a lot. Operating system goes 'blue screen of death', ATMs go down, store cash registers go down, airline computer systems go down. Facebook was offline yesterday. Malware, viruses,hackers everywhere. Software quality sucks.
Now up the risk. Faulty software will be controlling a multiple ton autonomous weapon. Will it kill the passenger, the pedestrian, or both?
Oh. Don't forget snow.