That may be the rational response under certain morals, but not everybody is rational always or has the same morals. Who is legally responsible for those Y deaths? Or, even if there was no death but the car broke a traffic law, how is that addressed? If the Y deaths are disproportionately a distinct class of people from X (say, pedestrians, construction workers), are they going to be eligible for some class action lawsuit? Or will it deepen class divides as poor pedestrians become bitter about being hit by self-driving cars carrying rich people? Are the "drivers" going to suffer guilt, wondering if there's something they could have done to prevent an accident? Are issues like this going to cause the law to require a human "standby driver" ready to take over, canceling many of the hoped-for advantages of self-driving cars? If drivers have legal or monetary liability, are they going to want to hand that over to a machine built as cheaply as possible and designed by overworked startup engineers? (perhaps the first generations at least.) Will the cars keep audit records to prove "who" was driving, etc.?

Maybe some of this is getting off-topic; I'm just musing on how complex the practical and societal factors could get.