She doesn't get it, not at first, why John would send another one of those damn machines back in time. He knows how much she hates them. At least this one doesn't have the face of the thing that killed his father.
"I'm not for you to understand," she tells Cameron. John gave it a name. Maybe he gave Cameron her a name for a reason. She knows now that she isn't there with John on Judgment Day. Or maybe she will be now. Dying from cancer is something she's never imagined. She always thought - still thinks - she's going to go down fighting.
She knows that much about herself. Sometimes she wonders how much she knows outside of the immutable - or maybe not so immutable - fact of Judgment Day.
Maybe this Terminator is a gift from her son. Cameron tells her things about John that no one else knows. She doesn't understand that about him. His fascination with the things that will destroy humanity is more than just the love of a good weapon that she works to instill in him.
Robots learn. She knows that Skynet will learn to hate, but she watches as Cameron discovers what it means to be a person. Some day, John will have the skill to change an unfeeling machine into someone who helps to save their lives. He will take the cold, hard data from Skynet and give it the potential for feelings, the chance to grow in ways that Skynet can't. Skynet won't ever let its creatures become anything but extensions of its will. Cameron will - already has - become something else.
Sarah can't bring herself to hate Cameron. She feels flashes of anger that this thing, this robot she can't stop herself from thinking of as a person, doesn't understand, but when the anger cools, she remembers teaching John the same things that Cameron struggles to learn in different ways.
She thinks that Cameron is the only grandchild her son will ever be able to share with her. She focuses on the next pull-up, and then the next, until her arms are shaking, not too much to defend herself from a surprise attack, but until she's too tired to worry at the puzzle any more.