What does Matt discover about Rosa?

In Chapter 15, Matt doscovers that Rosa is no threat to him..... that she was now an eejit with boundaries implanted in her brain..... a shell of her former cruel self.

Matt felt that old thrill of terror, as though he were still a small boy and she his jailer, but this woman posed no threat at all. The hard, bitter lines of her face seemed unconnected with anything going on inside.

Matt rode away from the house. The horse walked steadily. It would move in a straight line until Matt told it to go right or left, and it wouldn’t pass the boundary implanted in its brain. Like Rosa, Matt thought. For the first time he realized what a terrible thing it was to be an eejit. He hadn’t known any of the others before their operation. They were simply there to do boring jobs. But Rosa had been a real, though cruel and violent, person. Now she was merely a shadow with the life sucked out of her.