She could, if she didn't want to be able to live with herself afterward. She knows that. Tris got into her foster family's hands just in time to prevent her becoming inured. It takes a lot to shock Tris, when it comes to cruelty, but it doesn't take much to make her care. Her teacher, her mothers, and her siblings shoved her back into caring.

"Nonsense," she says gruffly, hand still in his hair. "Don't you know how satisfying it is to do something as opposed to just being impotently angry or worried?"