I think the black car symbolizes death, and since Seth has wanted the death of the "vampire", he watched her got on her car without any warning. It ended that Seth realized that the "vampire" is mortal after all, and he screamed in distress and confusion about all what's going on around him.

I love the film
The end truly is heart-rending and distressing; but as with much of Ridley's work it is full of symolism. As Seth screams and claws at the earth he has confronted his mortality and is terrified by the fragility of existence.
The car mainly acts as a symbol of death. Towards the end of the film one of the teens offers Seth a ride but he declines - to live another day.
I wish Ridley would make another film but he now gains such accolades for his kid's books I imagine that he will continue on that path

In my opinion, the ending is very powerful, but perhaps not so "terrible" after all. Screaming a perfectly sane reaction to all the madness around Seth, and something may be gained from it. It is a point of departure, either into more madness, or into healing.
About the teens in the black car, remember that Seth is just a kid and that no adult ever explains anything to Seth in a way so that he can understand it. How is he supposed to know anything about those teens? In his version of reality, vampires are the real threat, not teens in cars!