Same as when a selfdriving machine of any kind does it. The owner or the manufacturer. The owner if it was being operated/maintained in the wrong way. The manufacturer if it was an technical error.
... or the one who is run over, or no one at all.
Right now, when a car runs someone over, it is usually assumed to be the fault of the driver, even if he did nothing wrong. Blaming an individual is easy, but this could change if a large manufacturer is behind it. The manufacturer will have the data from the car sensors, skilled engineers to interpret them and a whole team of lawyers to defend them in court. If they can get away by proving that the victim was reckless, they will