It has 0.5 seconds until collision so no hope of stopping. Push the brake all you want, unless that car gets back in its lane, people die.

.5 seconds is both incorrect, and plenty for a car that sees and responds instantly to make a car-length braking correction.

A collision may not be avoidable, but every decrement to the speed will reduce its severity. It's the same idea as having crumple zones on a car body.

Do you have a specific example of this situation occurring as you've described it? If so, was there an ethical dilemma when a human was in control? If not, don't worry about it.